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'Reverse Replication" Rollback Strategy



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



Webert Lima
*Berlin, Germany*
*IRC NICK - WebertRLZ*