Say a witch who has Elements as patron gains a level up to level 14, and decides to take the Dimensional occultist archetype: what happens to her previous patron spells and hexes? And does she gain the archetypes all class features from level 1 to level 14 or how does it work? Thanks for help :)
Archetypes must be taken with the first level you have in a class, so if you weren't a Dimensional Occultist as a level 1 witch, you aren't one as a level 14 witch.
Now it's possible you could retrain your class to pick up an archetype if the GM allows, in which case you'd just rebuild your character with that archetype from level 1 all the way to your current level. But archetypes aren't "pick and mix" you have to take all the features the archetype has (and replace/alter whatever they replace) or you can't have the archetype. So a Dimensional Occultist can't have the Elements Patron, they have the Dimensions Patron.
Okey, I´ve been quite confused with this stacking of archetypes: so it´s possible to stack archetypes from different classes, but not from same class?
It *is* possible to stack archetypes. But all archetypes must be chosen when you first gain a level in a class, and they must not overlap. Specifically all the class features that one archetype alters or replaces must also not be altered or replaced by the other archetype.
So the Dimensional Occultist trades out their 2nd, 8th, and 12th level hexes, and forces a choice of a patron. One could combine this with a Mirror Witch (since all that replaces is your familiar) or Nexian Spellspy (alters familiar and replaces your 4th level hex) but not a Ley Line Channeler (which also replaces your 8th level hex.)
One could not be a Dimensional Occultist and a Winter Witch (the Archetype) since the Dimensional Occultist says "your patron is Dimensions" whereas the Winter Witch Archetype says "your patron can only be ancestors, deception, enchantment, endurance, moon, occult, portents, stars, transformation, trickery, water, winter, or wisdom."
Basically the rule of thumb is you can't replace the same thing twice. Some GMs will make allowances for stacking archetypes when two archetypes alter the same feature in a compatible way (e.g. both archetypes change class skills in a way that does not conflict) if you ask them, but by RAW you can't do this.
Thanks for great advice, this clarified a lot :)
But all archetypes must be chosen when you first gain a level in a classYou can also take archetypes later on, if they don't affect class features prior to that level.
Just to clarify this: Let's say you're playing a 1st level Witch. You're Vanilla (no archetype). When you reach level 2 you look at what options are out there, and decide you want to be a Dimensional Occultist. This is absolutely fine as the Dimensional Occultist doesn't modify anything until level 2.
If you went Vanilla Witch (no archetype at level 2, then when you reach level 3 you decided you wanted to be a Dimensional Occultist, you'd be out of luck. You've already gone past the point where the Dimensional Occultist changes the class so you can't choose that option anymore.
(Of course with the re-training rules you can swap things whenever you have the time and money to do so).
EDIT: I'm not super familiar with Witches, so I was just going off the spells your Patron grants you. If the Patron does anything at level 1 then I'm wrong, and you have to choose this archetype at level 1 =P