Although Latias is obtainable in many ways on OR, the soaring function is restricted to Latios for this game, and Latias for AS. The Eon Pokémon exclusive to either game is obtained in an in-game event, along with the Eon Flute used to summon the Pokémon from any location, and the appropriate mega stone. Even if you have a Latias, the Eon Flute and a Latiasite, and Latios is removed from your game, you will be stuck with Latios.