Thank you for posting a reply! I have a second Battle.net account. I only have one Steam account, and that's the account attached to my Bungie.net account. The Steam account is the correct account. I've successfully unlinked the first Battle.net account and want to now link the second Battle.net account that certainly has my D2 characters. Are you saying that I am unable to because I only had 1 chance to do so?
You cannot redo the migration using the same Steam account, as the PC Move is a one-time process for any platform accounts involved. You can still transfer the correct Battlenet account, but to do so, you'll have to use a different Steam account.