They are now updating the B side computer so it can manage the mission while they work on the primary. I wonder why this is not something that is kept up to date anyway. I can see keeping B an update or two behind A to prevent a single programming error taking both of them down. But after you are satisfied with A's software load, why keep B so far back-level that transition takes so much time. And since the computers are said to be identical, why the desire to move back to A?
They're running the same flight software, but the parameters are different. (A parameter might say, for example, how far to drive between autonomous visual odometry updates, or how big the bounding boxes around the arm should be when computing ChemCam laser safety.) There are thousands of these parameters, and they're not routinely kept up to date on the non-prime side (which has historically been the B side).
And while the computers are identical, the non-cross-strapped equipment isn't. For example, the B-side rear HAZCAMs are exposed to more radiation, because of the DAN instrument, than the A-side rear HAZCAMs, and are therefore expected to degrade faster. Switching back to the A side is, generally, switching back to slightly better equipment.