I think it's great it's heading to Steam, opening the game up to a bigger possible playerbase, but honestly not allowing existing owners a key for the game is quite a shame.
Speaking on the forum, they said this about it:
Can I play Albion Online on Steam with an existing account?
In order to play Albion Online on Steam you must purchase the game through the Steam store and create a new account. Once you have done this, you can either play on Steam with your new account, or login via an existing account.
NOTE: While ingame purchases of Gold and/or Premium status will be credited to the currently logged-in account, Epic and Legendary Packs purchased through the Steam store can only be used by the primary Steam account.
I actually find this a little odd, since I've seen hundreds of developers give out keys for existing owners, when purchased directly or on itch.io and other places. It strikes me as very strange that they're not able to do this. They already have their own login system, so it should just be a case of giving owners a Steam key, which is free for developers to generate. There's other games out there that let you easily link an email to a Steam account too, it's not difficult to do. Hopefully they will clear this up a little more.
No actual date is being given for the Steam release yet.