Something happened to me in-game which contradicted what I'd been told about the mechanics (so it may have been a change from a previous game); in a wild double battle, I have Swanna and Litwick out, against some random enemies (I forget what, but they didn't have any ability, etc, that would matter). I select Brave Bird on Swanna and Will-O-Wisp on Litwick, targeting different enemies. Swanna goes first and OHKOs its target, and Litwick levels up from the resulting experience gain, learning Purgatory as a result; I choose to delete Will-O-Wisp as it has four moves already. Then on Litwick's turn, it uses Will-O-Wisp even though it is no longer in its moveset. (I'd been told that it'd use the new move on the same target in this case, in previous gens; I'm not sure how accurate that is, though, but if it is accurate, this is a change in mechanics.)