RegisterReplicaDataset is available to arcObjects (.net), but is not available in Python. This is the only way to modify an existing replica, either to add ( https://desktop.arcgis.com/en/arcobjects/latest/net/IWorkspaceReplicasAdmin2_RegisterReplicaDataset.... )
Please make the functionality in IWorkspaceReplicasAdmin2 available via the arcpy library for Python 2 and 3, since this right now arcObjects is the only supported way to modify existing replicas.
* arcgis 10.0 - Add feature to replica - Geographic Information Systems Stack Exchange
* developer-support/arcobjects-net/unregister-datasets-from-a-replica at master · Esri/developer-suppo...