ContinuousTransition directly stops replica-set processes instead of using the existing ReplicaSetFixture.stop_primary() workaround. For the disaggregated replica set fixture, this lets the old primary observe the new SLS term while it still has an active primary append stream, causing an unconditional step-down and the old-primary to fail (here). 
 
As part of this ticket, we should first stop and wait for the old primary, and then restart it as a follower to retain the two-node election quorum, before stepping up the secondary.