I was just doing a triple battle, leveling my Pokémon up to level 50 to battle in the battle maison, and my Mamoswine leveled up and learnt Blizzard before it went for it's turn. If I had taught it over the move that I was using on that turn, what would happen? Blizzard, Icicle crash or struggle?
The new move that replaces the one you originally selected is used. So it would have used Blizzard. This has happened to me during power leveling too, so I can confirm personally.
edit: However, if the pokemon is locked into Outrage, it uses Outrage, even if you delete it. Maybe something similar happens if you are a victim of Encore? But I don't know which opponents even use Encore. I also know that Encore stops if you run out of PP of the afflicted move. So if you delete a move mid-battle, Encore probably ends and you use the new move. But that's just a guess.