So I'm pretty new to witches and I'm trying to figure out something. A familiar for a witch has spells "known"
Witch's Familiar (Ex)
At 1st level, a witch forms a close bond with a familiar, a creature that teaches her magic and helps to guide her along her path. Familiars also aid a witch by granting her skill bonuses, additional spells, and help with some types of magic. This functions like the wizard's arcane bond class feature, except as noted in the Witch's Familiar section.
A witch must commune with her familiar each day to prepare her spells. Familiars store all of the spells that a witch knows, and a witch cannot prepare a spell that is not stored by her familiar.
I realize that this might be flavor text as opposed to RAW, so I'm asking you guys.
Do witch familiars get new spells known when gaining a level in a prestige class that grants a level in spellcasting?
It would seem to me it would be horrendously expensive if it didn't. Barring finding familiars from other witches to learn spells (and good luck doing that for every level of the PrC), you'd have to buy a scroll of the spell each time. Seems it might be cheaper to release your familiar and bind a new one to get access to spells, correct? Seems to me that 2500 GP (assuming you enter a PrC at 5th) would be cheaper than buying high level scrolls.
It works largely the same as a wizards spellbook. that means that the 2 additional spells added are a class ability. Which means that when you take a non-witch/wizard level you don not gain additional spells known. The only difference I can see is if your new class advances your familiar you would gain the bonus spells, as the spells known are linked to your familiar ability rather than being separate.
I think your concern might actually be justified. Wizards and Magi (I think all prepared Arcane casters) also have this problem. Releasing and replacing a Familiar doesn't fix the problem any more than releasing and replacing your spellbook, and either one of these actions loses spells other than those you get automatically, unless you first back it up with Scrolls or a Stone Familiar (witch) or backup Spellbook (Wizard).
I think this is covered in a FAQ that I have seen, but I can't remember the proper search terms for finding it. You can find a copy of this FAQ in a sidebar of Mystic Theurge, but the link that is supposedly to the original FAQ just goes to Paizo's home page.
I think you'll just have to pay for or find spells.
Spontaneous caster explicitly don't have this problem (because if they did, they would be completely hosed, whereas prepared casters can get around it by spending money).
Edit: Semi-Ninja'd.
It depends on the prestige class. Usually it works as everyone has said: no automatic new spells known. But Winter Witch, for example, explicitly says that Winter Witch levels stack with Witch levels for spells gained at each level. So you would get your free new spells.