In a worst case scenario, one of the first-batch users gets hacked, starts spamming and Discourse decides that all users with the same registration IP are spammers, too. But I don’t have enough experience with Discourse’s anti-spam features (never needed them, thankfully) to tell for certain what would happen.
Anyway, you can run this in the Rails console inside the container (or, if you prefer that, extract the SQL bit and run it in psql):
User.exec_sql("UPDATE users SET registration_ip_address = ip_address WHERE registration_ip_address = inet '10.0.0.1' AND ip_address IS NOT NULL")
Simply replace 10.0.0.1 with your server’s public IP that was erroneously used as your users’ registration IP. All users who at this point still have that IP as their reg. IP will instead have their last seen IP address set as the registration IP.
(Caveat emptor: always make backups before manually running SQL statements over your production database.)