Yes, there's a faster way. Just replace the shelf in A.
Swarm keeps all the versions of your review. When you first request the review for A, it creates version 1 of the review. When you update the review (or re-shelve), it creates version 2 of the review. Etc. There's a horizontal slider in the Swarm review GUI, where you can navigate all the versions the review had.
Now the good thing: each of these versions is "implemented" using a Perforce shelf (in another workspace, managed by Swarm). Thus, you can always access and unshelve all the review versions. (You can even diff your versions using the two-dot button in the Swarm GUI.) To unshelve a specific version of your change, pick that version with the horizontal slider and then copy the shelf number where it says Change 1234 shelved into .... Copy the 1234, go to P4V, Ctrl-G, open change 1234, unshelve.