I.e ones who's master died and never got resurrected. Do they just gradually turn into normal animals again?
It's the sort of thing I wouldn't expect a consistent answer for. Basically, whatever your DM wants to happen should happen.
We do have a bit of text for witches, "If a familiar belongs to a witch that has died, it only retains its knowledge of spells for 24 hours, during which time it is possible to coerce or bribe the familiar into teaching its spells to another, subject to GM discretion." So, that implies that the familiar keeps being a familiar for at least 24 hours. But it's still uncertain if the familiar is still a familiar, just without spells after that 24 hour window.
Since they transform into magical beast I think the can continue to live like that he until they did even if the gm approved can take lv in class but can't take familiar