This PMOD mod adds a new option to Luminous Spring that allows pokémon to revert to their previous evolution stages.
Reverting item-based evolutions requires Heart Scales, but also gives the item back.
Any other type of evolution is free of charge.
Pokémon moves may change as a result of this process: any move that the pokémon could only learn in its evolved stage will be replaced by one it can still learn after devolution. It will also be impossible for the pokémon to relearn these moves until it evolves into something that can learn it again.