In principle, yes, although there are some concerns as to how this function works in SCALE.
Migration involves creating a new pool with the mirror of large new drives, and replicating data to it. Then you could destroy the old pool, and add the old WD Red (if they are Red Plus CMR) as a new vdev. But one would expect these drives to fail sooner than later, due to their age; simulatneous failure from both would lose the whole pool, so the question is whether you really need the extra 3 TB space and want to take the risk, rather than reusing these drives in a less critical use.