When you add a receptacle from a wall switch, is it typically necessary to replace the box? I'm referring to a new work box that's nailed to the stud. What is the best way to remove it? I have used a pry bar in the past, and that seemed to work okay.

 

Usually not necessary. If you are able to fish wire from existing junction box to the new location and the box is big enough to handle added wire, it is all good.

Best way to remove junction box is by cutting nails from the stud. I use a heck saw. Reciprocating saw will work too, but there is more risk of damaging wire if not careful.
Some junction boxes have bracket that is nailed to toe face of the stud and if that is metal junction box, you may have to cut the drywall. It is possible to cut it with grinder, but not easy. It is easier to patch drywall and paint in my opinion.

 

I have fished cables into the existing box without removing it. Or push the cable out of the box towards the new box and reach into the wall to pull it out.