The folks at Bungie have started the process to transfer your Destiny 2 accounts.
The process is pretty simple you go to bungie.net and click “get started,” then select the account you want to move over (you can move PS4 and Xbox accounts also) and link them as prompted. That’ll take you to an account review screen to confirm that this is something you actually want to do; assuming it is, click “continue,” agree to the terms, make sure you want to do this (again), confirm with Steam that this is all above-board, and then finally, you are done.
The deadline for transfers is 1st October after that your account is gone forever from battle.net so better do it now or regret later.