Here's how we decided this will work. When ORAS OU is released and the information regarding the game is all known, players can decide between facing off in either XY OU or ORAS OU. However, if players can't come to an agreement, ORAS OU will be the required metagame for them to play.
I'm sure you guys contemplated well about about this, I still want to throw my opinion in; On the one handside it's reasonable that you want to allow players to play ORAS OU, if they want to. But this was an XY OU tournament from the beginning on, so I feel like this should stay an XY OU tournament till the end, since you want to crown the best XY OU player.
Also I guess not everybody has the time, or mabe isn't in the mood, to build complete new teams for ORAS OU, a pretty random Metagame at this point. And all this in the short amount of time between the rounds, while the tournament is running. If I should be wrong and everybody is so excited about ORAS, let them play, but this shouldn't disadvantage the players who haven't got an ORAS OU team yet. So maybe let XY OU be the "official" mode and ORAS be optional. =)