I've always found the 'pet evolves, personality shifts' trope a bit lazy if it's not done carefully. Take the dragon hatchling that goes from cute to terrifying—too often, it just flips a switch from loyal sidekick to aloof god-beast. Feels like a missed chance to show a more blended, conflicted identity. I prefer when the evolution is tied to a moral choice for the owner, like in some LitRPGs where you pick a branching upgrade path that locks out certain companion behaviors forever. That reflects how real growth closes some doors while opening others.
That said, I'll admit I'm a sucker for a well-executed behavioral twist. In one indie fantasy I read, a character's feline familiar evolved wings and started hoarding shiny objects obsessively, which actually created hilarious plot obstacles instead of just making it more powerful. The core mischievous personality remained, just amplified in new ways. It felt more organic than a total rewrite.