In an interview with nTower, Junichi Masuda revealed there would be no patch for Pokémon X and Y that would add compatibility for new Mega Evolutions that will be introduced in Pokémon Omega Ruby and Pokémon Alpha Sapphire. It should be noted that there is no technical reason for not adding compatibility into X and Y via a patch as all the infrastructure to patch the games is available as its the same game engine and generation and there has already been two patches for X and Y which shows it can be done. Nintendo and GAME FREAK have always required people purchase the latest Pokémon games to play in the current tournaments and this is probably the only reason there will not be a patch.

nTower Interview with Junichi Masuda and Shigeru Ohmori Translation

nTower: Will Pokémon Omega Ruby and Alpha Sapphire be compatible with the games Pokémon X and Y? Can the new Mega Evolutions be used in the Kalos Region (for example with an update)?

Masuda: We do not plan to release a patch for Pokémon X and Y to update the games that effect. There is a separation of what you can experience in Pokémon X and Y and in Pokémon Omega Ruby and Alpha Sapphire. Therefore, we want to separate the games to that effect.

Pokémon Global Link Changes for X and Y

On October 15th, 2014, the Japanese Pokémon Global Link website added a page to discuss some of the upcoming changes that will affect X and Y with the release of ORAS.

Source: http://3ds.pokemon-gl.com/information/ac78b42b-6759-441d-8da1-5f3116defe06 and http://www.ntower.de/index.php?page=Report&reportID=271