>If a Pokémon levels up mid-battle and a new move is learned in place of Rollout while it is still being used, the Pokémon will continue using Rollout in spite of having forgotten it.
So yes, Rollout will continue to be used until it ends or misses. I am still testing to find wether this is still so in gen 6, but I will give the results as soon as I can.
Hope I helped!
Edit: I tested it, and found that it is still the same in X and Y. Sorry I couldn't test it immediately, but I had to set things up and the game wasn't exactly cooperating when I finally did get everything ready, if you know what I mean.