Improve changelog with migration note

Author: sbernauer

CHANGELOG.md

@@ -17,7 +17,8 @@
 - Support setting `clientAuthenticationMethod` for OIDC authentication. The value is passed through to the Flask-AppBuilder config as `token_endpoint_auth_method` ([#719]).
 - BREAKING: Rename `EXPERIMENTAL_FILE_HEADER` and `EXPERIMENTAL_FILE_FOOTER` in `superset_config.py` for arbitrary Python code to `FILE_HEADER` and `FILE_FOOTER`  ([#719], [#721]).
 - Use an internal Secret for the Superset `SECRET_KEY`.
-  Going forward, the operator will automatically create the Secret in case it doesn't exist ([#722], [#754]).
+  Going forward, the operator will automatically create the Secret in case it doesn't exist.
+  If your `credentialsSecret` at the time of upgrading points at a Secret with `connection.secretKey`, we  will automatically migrate your existing `SECRET_KEY` to avoid interruptions ([#722], [#754]).
 - BREAKING: Implement generic database connection.
   This means you need to replace your simple database connection string with a typed struct.
   This struct is consistent between different CRDs, so that you can easily copy/paste it between stacklets.