During Upgrade Admin was usually offered chance to Copy Custom Content Center Libraries to Partition of new Release. That seems to be missing when upgrading to 2023.2.1 - and we have opened a specific ticket for this issue.
But the natural next step would be for the Upgrade process to offer automatic creation of replication of these "new" CC Libs to same Connected Workgroup as the "old" versions. An extended bonus routine would also add the option to remove the replication of the "old" CC Libraries - similar to the Copy Content dialog show earlier in Migration.
In our case with 27 Custom and 10 Std. CC Libraries replicated to 9 Subscribers - this means 4 hours of our lives we will never get back during each Upgrade - plus the added risk of manual errors etc.!
Best regards
Uffe Bak