How to change stats & cr when npc turns into demon?

So umm, so if quasit with class levels steals powerful soul and uses it to get promoted into stronger type of demon, how do I handle its stats changing?

And if 0hd race multiclass with prestige class turns into vrock, what the heck is cr(and again not sure how to handle stats changing)? Like, I have no clue what vrock with 4 cleric levels & 2 fighter levels would have as cr, and thats before adding divine scion levels

First question: sounds like you'd just use the stats of whatever Demon the Quasit turns into, unless I'm missing something?

Second Question: You're asking about Monster Advancement (Linked here - Scroll to the bottom for Class levels). It can be a lengthy process, but it's relatively straight forward:

First, put the Vrock's monster entry onto a Word doc, or notebook paper (Word doc is easiest, since you're going to be changing a lot of things, and backspace leaves less mess than an eraser). Copy the stats down verbatim. This is your base creature. Now, we're gonna ass 2 levels of Fighter. Step 1 of adding Class levels on the linked page is to determine the base creature's combat role. An easy way to determine this is to reference this chart on the Monster Creation rules. Looking at the chart, we can see that a Vrock is just under the bar for AC and HP expected of a CR 9 creature, and way under the bar for high attack roll, and is smack dab in the middle for high and low average damage. However, it has average to above-average save DC's for its abilities. Taking a closer look at them, a Vrock has some pretty potent abilities, especially when paired with other Vrocks. With all of this data, we can determine that a Vrock's Combat Role is Special (generally tough in combat, but intended/expected to rely on its special abilities to meet its full potential). A fighter is most certainly in the Combat role. This means we're adding unsynergysing class levels, so its CR will only go up by 1 for every 2 levels of that class (until we have Class levels meeting/exceeding its original CR, but that's not going to happen here).

When adding Class levels, before you make any other changes, you always adjust a creature's ability scores with the following (however you like): +4, +4, +2, +2, +0, -2. Remeber to adjust the DC's of its Spell-like Abilities (Cha), its Dance of Ruin (Cha), and its Stunning Screech (Fort) with any changes to the corresponding ability score modifiers. OK, NOW we're ready to add the Class levels. Just take the numbers, and treat it as if it were a normal character. The BAB, base saves, HP, proficiencies, and skill ranks go up exactly as a Fighter's would. Remember that the standard Vrock has 9HD, so your first level of Fighter gets a Combat feat, but not a standard feat, since its gaining an even-numbered level. Its second Fighter level, however, gains a standard AND Combat feat.

Now it's time to add your 4 Cleric levels. The Cleric fits into the Caster role, so again, we're only adding 2 to its CR (for a total increase of 3). We already primed the Vrock for Class levels when we started with the Fighter, so this time is going to be easier; just add BAB, base saves, skill ranks, proficiencies, HP, and spells per day exactly as if it were a normal, multiclassing character. Make sure to also add your Domains, Channel Energy, etc.. Keep in mind, you're still gaining feats on even levels, because it's on the even levels that the Vrock's gaining an odd-numbered HD. Speaking of HD, don't forget to add 1 to an ability score when you gain that 12th HD! Also, remember that the DC's for its Special Abilities (Dance of Ruin and Stunning Screech are adjusted by 1/2 its HD, and we just added 6 HD, so add another 3 to the DC's of those abilities.

The final monster's CR should line up with a CR 12 creature on the Monster Creation chart. A CR 12 Monster gets 9,000 gp in loot, as well. Feel free to incorporate that into its gear, with items such as a Ring of Deflection, Amulet of Mighty Fists, etc. mixing in some non-combat loot like gems is usually the way to go, so you're not over-gearing your monster. Just use the gear to try to fill in any holes the creature may have gained in its offense/defense during the Class level modifications. Its Caster Level for its SLA's doesn't go up with its HD.

I hope this was helpful to you, and good luck!

Well quasit in question had witch as class and I was planning to turn her to Vrock too <_< I might remember wrong right now, but pretty sure she had better stats for casting than default Vrocks do. BTW, with witch class it was "2 witch levels = +1 cr" with vrocks, right? Sorry if I got it wrong, I'm really easily confused by monster advancement rules and I'm really bad with creating statblocks...

But ok, so vrock cleric 4/fighter 2 is cr 12. So if I add divine scion levels, how much that adds to cr per divine scion level? Like I'd imagine that 10 divine scions levels would be way over kill for level 16 party so I want to figure out how much I should add so that this encounter with two "oh you knew them previously and they now return as vrocks" wouldn't be too overkill cr wise :' D

But yeah, thank you for advice, it was helpful though I'm still bit lost..

CR for class levels is more of an art than a science, I'm afraid.

Converting attributes shouldn't be too hard.
A Quasit has
Str 8, Dex 14, Con 11, Int 11,Wis 12, Cha 11
A Vrock has
Str 21, Dex 15, Con 25, Int 14, Wis 16, Cha 16
So to convert a modified Quasit to a similarly modified Vrock take the current stats and add:
Str +13, Dex +1, Con +14, Int +3, Wis +4, Cha +5

Just to note, the quasit in question was originally level 3 witch with these stats: Str 8, Dex 18, Con 13, Int 15, Wis 10, Cha 13 <_< And none of stats are from buffs or items

But yeah, that method sounds like it would make sense, so I guess I'll go with that unless I get another idea

Well quasit in question had witch as class and I was planning to turn her to Vrock too <_< I might remember wrong right now, but pretty sure she had better stats for casting than default Vrocks do. BTW, with witch class it was "2 witch levels = +1 cr" with vrocks, right? Sorry if I got it wrong, I'm really easily confused by monster advancement rules and I'm really bad with creating statblocks...

But ok, so vrock cleric 4/fighter 2 is cr 12. So if I add divine scion levels, how much that adds to cr per divine scion level? Like I'd imagine that 10 divine scions levels would be way over kill for level 16 party so I want to figure out how much I should add so that this encounter with two "oh you knew them previously and they now return as vrocks" wouldn't be too overkill cr wise :' D

But yeah, thank you for advice, it was helpful though I'm still bit lost..

I'd just copy paste my instructions above for the Witch - take a Vrock's stat block, add [+4, +4, +2, +2, +0, -1] to its stats in any way you want, and then add 3 levels of Witch, keeping track of your total HD. The final result should be between a CR 10 and 11 (take a close look at the Monster Creation chart to figure it out for sure).

Adding levels of Divine Scion to the multiclassed Vrock will add 1 CR for every 2 levels, since the Vrock was CR 10 before it gained any classes with the Caster role. it has 4 levels of Cleric, which added 2 to its CR (CR 12). At this point, the creature has "9 CR's" from its Default stats [in a Special role], 2 levels from a Combat Role, and 4 levels from a Caster role. It's CR before any Caster-role classes is effectively 10. According to the Monster Advancement rules, its CR goes up by 0.5 per Caster-role level until it has a number of such levels equal to its previous CR (which is 10). If you add 6 levels of Divine Scion, it will have those 10 levels (Cleric 4, Divine Scion 6), and will be at CR 15. From here, each level of Cleric or Divine Scion you add will increase its CR by 1 instead of 0.5.

Adding class levels to monsters can get a bit messy, especially when you multiclass. To clear things up, here's your road map:
Vrock [CR 9] >> 2 levels of Fighter [CR 10] >> 4 levels of Cleric [CR 12] >> 6 levels of Divine Scion [CR 15]
At this point, we've added [+4, +4, +2, +2, +0, -2] to its ability scores, and added all the HP, base saves, skill ranks, proficiencies, spells per day, and class abilities of a level 2 FIghter, a level 4 Cleric, and a level 6 Divine Scion. Our new total HD is 21, which increases its Dance of Ruin and Stunning Screech DC's by 6 each, and grants it 6 new feats and 3 +1's to any of its ability scores.
From here, if you want to further increase its CR (and I'm guessing you do, since you were asking about a group of level 16 adventurers), you can continue to add levels of Cleric or Divine Scion to increase its CR by 1 per level. If you went to level 10 in Divine Scion, this would give you a CR 19 creature.

Ok, I think I'm starting to get this better. So in case of vrock witch, up until 10th with level each level would raise cr by half, but after that when class levels reach cr 9 it starts increasing it per level?(and vrock witch level 10 would be cr 14?)

Anyway, thank you a lot for help :'D I'll ask again if I run into corner, but I'd imagine now after understanding that hard part would be just figuring out equipment.

Glad to help!

That Witch Vrock is going to be between CR 14 and 15, since it technically crosses the "un-synergysing levels = CR" threshold at level 9, which leaves it at about CR 13.5, then adding a 10th level adds a full CR to 14.5. The Cleric Vrock had those 2 levels in Fighter to make the numbers a bit easier to work with, having a CR 10 base to work with instead of a messy CR 9. But you do have the idea, I'm glad I was able to explain it coherently enough for it to make sense!

Uh, just realized when reading bestiary that last page actually lists Vrock's role as combat so wondering if that changes calculations much x-x; Well, it probably won't. My biggest problem right now is just picking spells, I'm really bad when it comes to prepared casters :D

I would only really fill out the high level spells. In a fight he will probably use all the top level or two spells asap. For first or second levels, just pick one or two that are clearly helpful. Usually long lasting buffs (1 min+/lvl). You're unlikely to caste beguiling gift when disintegrate is still an option and the NPC won't live long enough to get to their 5th 1st level spell of the day.

Look at how it is done for the Simple Class Templates from the Monster Codex

Remember, the players will never see your 100% by the book, totally accurate stat block, so it doesn't have to be perfect.

Another suggestion would be using Pathfinder Unchained monster creation spell lists (LINKY) Just pick a spell list and use that.

So yay, today is the day to finally use these. I'm pretty worried though whether I got stuff right or not..

So guess vrock's cr just rises by one since the fighter levels are full ones instead of half? Or would be it be two... Ah well that doesn't really matter that much besides for exp. I'm not really planning to use fully equip them anyway so doesn't matter if they have lower cr's wealth for equipment

I am kinda sad I didn't ask tips for new feats before since feats are much harder to figure out than spells and spells were hard too :'D