IMO, this move is more about getting a guy who is more in line with what Fisher wants his Safeties to be, and that is interchangeable. When they committed to McLeod in this scheme they have almost been forced to keep McDonald in the box exclusively. If and when Barron gets up to speed, having them both on the field, will allow them to play more of a right/left side Safety and switch up the looks they give.
The other thing that might be influencing this sort of move, is the apparent step back Ogletree has made in coverage. He just seems to be overwhelmed with all the different things they are asking him to do this year, and this will free him up to be more concerned with playing the run, and not so much about covering TEs.
I also think it speaks to the injury concerns at the position. McLeod and Davis are more than likely to miss a game or two, which leaves them seriously thin back there. No guarantees that Jenkins AND Johnson will be ready this week. So that could keep Joyner locked in at the Nickel.