pg_upgrade requires that if any functions are dependent on a shared library, this library must be present in both the old and new installations before pg_upgrade can be executed.
To minimize the risk of any upgrade issues (particularly if an upgrade to a new major repmgr version is involved), we recommend upgrading repmgr on the old server before running pg_upgrade to ensure that old and new versions are the same.
Note: This issue applies to any PostgreSQL extension which has dependencies on a shared library.
For further details please see the pg_upgrade documentation.
If replication slots are in use, bear in mind these will not be recreated by pg_upgrade. These will need to be recreated manually.