I love how everyone has blown this way out of proportion.... You can only NOT trade MEGA from R/S to X/Y. This makes sense since though games don't have those Pokemon at all in them. What would even be the point of trading down anyway? If you are a serious Pokemon trainer that goes out for tournaments you would update either way. If you are just a casual why do you care that you can't trade back the likely 20-30 Mega stones. X/Y have 0 code for the new megas so it isn't about patching it is about actually updating the whole game with the new ones which is dumb since you need the newer version to even do that.