Bidirectional sync between Windchill RV&S and Codebeamer during a phased migration?

Does anyone know of an existing tool or pattern for bidirectional syncing between RV&S and Codebeamer? We have a client planning to move from RV&S to Codebeamer, but not until an upstream team moves from DOORS to Codebeamer. They have a process for syncing between DOORS and RV&S, but once the upstream team moves off DOORS, they will lose their tracing. We need a solution for the gap until we are ready to migrate RV&S.