When upgrading your Tor relay, or moving it to a different computer, be sure to keep the same identity keys (stored in "keys/ed25519_master_id_secret_key" and "keys/secret_id_key" in your DataDirectory).
If you are a bridge operator, also make sure to keep pt_state/. It contains data required for your bridge to keep working with the same bridge line.
For simplicity, just copying over the entire DataDirectory should work too.
You may wish to keep backups of these identity keys, plus pt_state for a bridge, so you can restore the relay if something goes wrong.