Strength is an issue, and there are several kinds of old work boxes (also known as "Cut-in boxes") The strongest is the side-grip metal box. They used to be available only through commercial electric supply houses, but lately they have them at Home Depot. At a glance, they look like regular switch boxes, but they have a screw on each side of the box that when tightened pulls up the end of a metal flap that bends outward and grips the drywall from behind. The gripping part is about 2" wide, so it's very strong.
I'd say something unkind about advance planning, but no matter how well you plan, needs change. In my business we have to deal with this all the time. New occupants want their phones and computers in different places, and tearing out all the walls is very inefficient!
I tried to add a picture, and it didn't work. Search for "Raco Old Work Box". There's a pretty good picture on Grainger's web site, though it shows the box with the grippers deployed, not flat on the sides of the box as they are when you buy them.
Hard to explain, but when you hold one in your hands, you'll understand in two seconds flat.