Couchbase Server users looking to upgrade from version 4.x to 5.0 can benefit from transitioning from equivalent indexes to index replicas, which provide improved manageability. The process involves creating new index replicas with a specified number of replicas, ensuring sufficient resources are available, and then dropping the old equivalent indexes once the replicas are built and online. This migration allows N1QL queries to utilize the new replicas without application downtime, although any queries using the 'USE INDEX' directive will need updates to reflect the new index names. While both equivalent and replica indexes can coexist in version 5.0, index replicas are recommended for better efficiency. Although the version 5.1 doesn't allow increasing the num_replica without recreating the index, the ALTER INDEX feature introduced in version 5.5 provides more flexibility in changing index placements.