To be clear - nothing is actually being migrated today. You're just linking your Steam account so that we have a place to send your Blizzard stuff when we do the migration right before Shadowkeep is available. It also puts you in a good place in case you're Cross Saving tomorrow, so everything will just work as you expect on October 1st without having to come back to bungie.net that day and mess with some settings to get access to your Blizzard characters on Steam when you just want to go check out the Moon.