In the post refresh plan, modifying external ids to a known valid set of values for UAT is the best way to resolve the issue of objects that have external ids from production no longer pointing to valid external ids in the UAT environment. This way, the data integrity and consistency across the integrated systems can be maintained. Letting UAT point to production integrations and rolling back each transaction after it finishes is not a good practice, as it can cause data loss or corruption in production. Deleting all the data and using an automated testing tool to create new data across all the systems in UAT is not feasible, as it can take a lot of time and resources. Masking the external id so nobody can see the production value is not a solution, as it does not address the underlying problem of invalid references.