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Carefully cut with a Sawzall between the box and stud to slice through the nails (use a bi-metal blade). You will want to brace the Sawzall with your hand so you don't mark up the wall. It should'nt take much to cut the nails, just be carefull of the wiring of course. To replace, throw in a remodel box OR just use any old box and screw it to the stud with drywall screws from the inside of the box.

But of course a bigger screw may be easier if you don't care about looks =)