I know the holes won't line up with the existing, but the fixing holes on the plate can be drilled and fixed through the opposite corners in the back box. Put a bit of sleeving over the screws to eliminate sharp edges it's a lot less mess than changing the boxThe 4 holes won't line up with the holes in the sockets/existing boxes though AFAIK.
It is possible to change metal back boxes without re-plastering, patience is a virtue (that I don't have these days), but it can be done, I have done it and seen it done.
This is one of those PITA jobs.