I think you may need to explain what you mean a bit more. I have friends at the table who are "car guys", but I'm not.
Taking a stab at what I think you mean though; you seem to be asking about game mechanics/class features that would support or encourage one in modifying an existing AC into something different from its original form/function without necessarily knowing if the change is going to "be better" or work as you thought it might. If that's the case, the it would seem to me that the first answer is just play style. You don't have to have an AC or other pet creature die in order to replace it. You can choose to dismiss or abandon a pet feature for many reasons, but you aren't guaranteed a replacement until you've spent the appropriate resources (time, money, rituals, reagents) to get one. If you play a character who tries to find the best AC in every new land or terrain, that could be one way to tinker.
If the goal is more to modify and keep one AC, then I would look at any of the feats or archetypes that allow you to add Eidolon evolution points to and AC. With appropriate spells or magic items, those points can be shuffled around for various abilities, allowing semi-constant tinkering.
Last thought for now, would be to look into the handle animal skill, and search for ways to allow an AC to learn more tricks than normal, or to retrain the tricks they know for others. Then your tinkering with how the "car" works. Depending on how much downtime your character would have, that may be the closest example to, the car is still a car, but I this time I messed with the wheels instead of the engine. . . . Sorry for the clunky metaphor. I reaaallly don't know much about automotive mechanics.
