The possibility is about to become a reality. This morning, the Aion community team announced that the service will be live in just a few weeks. Details are few at the moment, but the core questions of "when and how much" have been answered. Daevas can start switching servers on April 27th for a fee of $24.99.
Players will almost certainly have questions regarding the specifics of this service, so the community team is working on a FAQ to be released closer to the 27th. You can view the notice on Aion's announcement page.