Whilst I know that this is mostly up to the DM, but the Find Familiar spell in D&D means that you summon a spirit which take the form of your familiar. You can change form, but they always take on the stats of that form. I presume, however, that you have a relationship with that spirit/familiar which is going to be very strange if their intelligence and wisdom changes. Cats, for example have an animal Intelligence of 3 and Wisdom of 12. Pseudodragons, as can be summoned by Warlocks, have an Intelligence of 12 and Wisdom of 10. The spell says that if you cast the Find Familiar spell when you already have a familiar, it can take on a different form, ergo it is the same spirit. So does it’s character change? In which case, is it going to hate going from an Imp (Int:11) to a Rat (Int:2)?