The following steps should work:

1. Stop your Coder deployment.
2. Runcoder server postgres-builtin-serve in a background terminal.
3. Runcoder server postgres-builtin-url and copy its output command.
4. Runpg_dump <connection-string> to dump the database contents. Then restore that content to an external database, and start Coder pointing at the new one!

Let me know if something needs to be changed; happy to help!