Any purchases made through Activation are supposed to transfer to Steam. I believe all year one content is supposed to be in the F2P version, so you're really only buying the upcoming expansion if you purchase on Steam. They just don't seem to be playing up the F2P aspect during the pre-order phase (obviously they would rather get a bunch of pre-orders from people unaware there will be a F2P version at launch). Accounts and progress should also transfer over. It sounds like they are working pretty hard to ensure nobody ends up worse off over the transfer.