So far only one has, that is Ampharos will be Electric/Dragon instead of just Electric.
Yeah, I just looked at Serebii to see. I think it'll only apply to Pokemon that have one type before that stage. --- ''Your drill is the drill that will pierce the Heavens!" http://i.imgur.com/MkNCuK6.jpg http://myanimelist.net/profile/Heaven-Piercing
@ linkhawkins most of the pokÚs that have been revealed with megas are already duel types, with exception of ampharos and mewtwo. I hope they won't be OP, I also agree that there should only be about 50-100 with megas --- I was an adventurer like you, until I took an arrow to the knee.