I'm currently working on migrating a Galera 10.3 cluster (A) to Amazon RDS. (B) 

In order to minimize impacts and downtime, the plan is to have a standard MySQL Slave on RDS (B) to sync data, then do the switch to it and stop the replication A->B.

The main concern is a catastrophic scenario on B where a rollback to A is needed. How to bring back only new data that now sits on B to the old cluster A?

One approach I saw is to, before switching applications to the new server, stop the replication A->B and enable replication B->A.

This is my question here. Before the switch, the data will be the same on A and B but B will not be part of the cluster A, so things like a binlog position change.
Is it possible to enable this opposite replication B->A without having to extract a whole dump of B and restoring it on A ?

Thanks