ArcGIS Online will not allow you to overwrite an existing hosted feature service if replicas exist. ArcGIS Online also will not allow you to programatically compile a list of all of the existing replica IDs for a hosted feature service and then use that list to unregister them all. There needs to be a way to either programatically gather all of the replica IDs and unregister the replicas, or provide an "Unregister All" button on the feature service page so that users do not have to spend hours copying and pasting each individual replica ID into the one-at-a-time "Unregister Replica" tool.