I wonder how bumblebee upgraded himself, in TF3 his altmode is much more modified camaro than in the first and secound movie. I doubt he scanned a new camaro, si also dont think that bumblebee was tuned with earth stuff, maybe bumblebee decided to add some cool features and created the spoiler, new hood etc. by himself, that would mean that movieverse transformers can tune itself ?? What do you guys think.
Personally I think it is a maturation process: as they age they have changed features just like we do. Spoilers, hoods, wheels, etc. are just how they choose to change them with their disguising. My non-fanboy answer is that Hasbro knows we will not buy the same toy 3 times so they have to change some features of them to keep us buying. Of course this is not counting the numerous repaints that follow the original.
I think it's just Bumblebee being Bumblebee. I always figured his altmode changes in ROTF were just him keeping his look fresh. I like to personally think that Mikaela's crack about his altmode made him self-concious about his appearence, so now he updates it regularly and obsessively.
I actually like the idea that a transformer could make modifications to their alt-modes if they wanted to. Kind of like in the same way humans will try a new hair style or something.