Given the vast amount of content produced by Paizo for Pathfinder 1, I have to admit some trepidation when PF2 was announced - especially given the rather unpleasant experience in trying to convert between PF1 and 5E at times.
Thankfully, Paizo seem to have nailed this reasonably well - DC and Saves diverge somewhat, but that is inevitable given the change to how they are calculated. Some of the numbers naturally need to adjust, but on the whole the tone, feel, challenge and abilities of the creatures are retained.
There are a few notable exceptions, such as most outsiders losing greater teleport at will, and instead getting dimension door... but frankly I think in that instance that is for the better. Having that escape-and-return-to-murder-you-in-your-sleep ability attached to a vast array of common foes was always unnecessary and often ignored by GMs who just wanted a straight-up-fight.
Some examples:
The humble boar is CR2 in Pathfinder 1, and Level 2 in Pathfinder 2
AC: 14 (PF1) vs 15 (PF2)
HP: 18 (PF1) vs 30 (PF2)
Attack: +4 (PF1) vs +8 (PF2)
Damage: 1d8+4 (PF1) vs 1d8+4 (PF2)
Saves: Fort+6, Ref+3, Will+1 (PF1) vs Fort+4, Ref+2, Will+1
This seems a big divergence - double the hit points and +4 to attack, however it is also understandable: In PF1 PCs had to wait to get multiple attacks, while in PF2 they have 3 actions - each of which could potentially be an attack - from 1st level.
The higher attack bonus is more notable, but this ties into three things: Armor gives a lower AC bonus at 1st level; PCs add their level to their AC; and PCs not only have (roughly) 24-25pt gen by standard, but have bonus hp from their race.
Using the PF1 boar RAW in a PF2 game would perhaps be a bit too easy, but not game breaking.
AC: 17 (PF1) vs 15 (PF2)
HP: 16 (PF1) vs 35 (PF2)
Attack: +5 (PF1) vs +8 (PF2)
Damage: 1d8+3 (PF1) vs 1d8+3 (PF2)
Saves: Fort+2, Ref+4, Will+1 (PF1) vs Fort+7, Ref+6, Will+4
So in PF2 the bugbear has lower AC - which is apt as the PF1 bugbear had much higher AC than normal for its level - but higher hp. Its damage is similar, though it also has a much higher attack bonus. It also has bigger saves.
AC: 21 (PF1) vs 25 (PF2)
HP: 153 (PF1) vs 233 (PF2)
Attack: +20 (PF1) vs +20 (PF2)
Damage: 4d6+22 (PF1) vs 3d12+7 (PF2)
Saves: Fort+15, Ref+12, Will+10 (PF1) vs Fort+19, Ref+15, Will+14
Hello! This is an interesting case: The PF2 version is 1 CR higher, and comes with high AC and HP, but unlike the lower level creatures, its attack bonus is the same. An interesting thing is the damage; the PC eating PF1 T-Rex's average of 36 damage on the bite is significantly more than the PF2 average of 26.
If we ran the PF1 T-Rex in PF2 mechanics, we'd have a significantly more fragile creature, but with extra damage - more of a glass cannon, essentially - compared with the PF2 version. Though I'd argue it wouldn't be unworkable, merely different.
AC: 23 (PF1) vs 28 (PF2)
HP: 142 (PF1) vs 165 (PF2)
Attack: +21 (PF1) vs +20 (PF2)
Damage: 3d6+15 (PF1) vs 2d12+14 (PF2)
Saves: Fort+14, Ref+4, Will+9 (PF1) vs Fort+18, Ref+14, Will+16
Interesting! The PF2 version of the iconic fire giant has more AC and slightly more hitpoints, but roughly the same attack and damage. The saves are also much higher, but that is expected given the way DCs are calculated in PF2 (full level, rather than half).
The big thing here is AC - using a PF1 fire giant statblock in a PF2 game would be akin to running a PF2 fire giant who forgot his armor.
AC: 36 (PF1) vs 44 (PF2)
HP: 370 (PF1) vs 460 (PF2)
Attack: +31 (PF1) vs +35 (PF2)
Damage: 2d6+13 (PF1) vs 6d8+18 (PF2)
Saves: Fort+29, Ref+17, Will+25 (PF1) vs Fort+30, Ref+30, Will+33
We're now in crazy land. At this point, a PF2 Balor is a bigger, badder demon than its PF1 counterpart.
Overall, the numbers do fluctuate a little (as demonstrated in the above), but on the whole the PF1 numbers actually unusable or hard to adapt to get 'on feel' for PF2.
Being able to grab any one of Paizo's wonderfully horrible PF1 creations, give it a quick massage and throw it into the PF2 ring is amazing, as it gives us a back-catalogue of content that is easily usable.
I'm keen to hear how others are finding converting Pathfinder content to the new mechanics.
As an addendum;
In PF2, assuming PCs fight creatures roughly equal to their own level, they'll need to defeat 25 such creatures to gain a level. This is regardless of what level you are.
In PF1, it's slightly more complicated, but depending on your level it sits between the Medium (20-21 CR=Level creatures) and Slow (28-33) track.
I haven't crunched the numbers for other combinations of encounter difficulties... but as a baseline, we can expect advancement from combat to sit as being slightly slower than the standard Medium track.
that's reassuring. even at high level it seems, I can switch PF2 monsters from their future AP with their PFOriginal counterparts fou our PF games
the antagonistic NPCs still give me headaches, though
Overall, the numbers do fluctuate a little (as demonstrated in the above), but on the whole the PF1 numbers are not actually unusable or hard to adapt to get 'on feel' for PF2.
Memo to self: Make sure you type in all the words in the sentence, otherwise the message gets wonky.