Well, first off, the official term (yes, there actually is a word for this) is devolution. There's even a page about it on Bulbapedia; http://bulbapedia.bulbagarden.net/wiki/Devolution
I'd be kinda glad that GameFreak decided to implement devolution in the games, though I'd be kinda pissed if we could never evolve them again. Suppose someone has a female Glalie, and decides they want her to be a Froslass instead? I'd prefer a devolution item that devolves the Pokémon, but lets it keep the moves it had, and allows it to evolve again (provided it's able to meet the requirements- that is to say, as long as it's not something like a level 100 Wynaut that can't level up again).
Question- would your idea allow for a Light Ball to work on a devolved Pikachu?
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