As soon as the Realm information is out, every Legion will most likely start a debate to decide which one they will join. Members' opinions will be heard and possibly, votes casted. As long as people give good reasons for picking a Realm over another, he'll be heard.
About people leaving Legions for other reasons, unfortunately no one can control it. I guess that you have to prove your members that it's not worth to leave the Legion because of something else. Friendly environment and active friendship always helps meanwhile.
Good luck picking your guild.
