>>17894544The anime treats Overgrow, Blaze, and I would assume Torrent, as abilities that activate after enough damage and emotional trauma occurs, with a very visible effect and a dramatic increase in power.
Ash's Sceptile did it quite a bit, and his Infernape's Blaze ability (Fire version of Overgrow) was basically it's huge Thing.
The fact that Chespin has Overgrow instead of Bulletproof though makes me think he'll stay a Chespin rather then evolve. Then again if he went all the way to Chesnaught the Anime could just excuse away his physical shield on his back being used for Bulletproof while still having him Overgrow in a pinch.