I think MC cable would be ideal, but since the boxes I see are usually just the standard NM type with no clamps, I'm not sure it could comply with 314.3 or 330.40.
Changing out the box would likely be major hassle too, since they normally use either an MS or B-bracket, sometimes called a flag, that mounts on or over the face of the stud.
I would most likely have a hard time finding a suitable old-work box with cable clamps that's big enough to fill the hole left by a new-work NM box after removal anyway.
I suppose this lends itself to using a two-gang replacement box, but that may not be to the homeowner’s liking. I'm not sure if a Smart box would work in this instance either, since the remains of the original boxes bracket might interfere with the mounting of it.
It's seems that metal studs have been used in construction for quite a long time now and manufactures should be offering more listed products to deal with retrofit situations like these, not just new installation issues.
The only items I actually know of that are available for metal stud electrical retrofit work are just the same standard old-work boxes, fish tapes and a few inadequate sizes of Diversibits designed back in the mid 1980's.
