This 50s Junction box does not budge. I want to replace both the box and the wire, but neither is moving. The wire won't move because it runs horizontally before the vertical. I'd like to remove the box so I can replace the wire. There appear to be no screws, bolts, or any fasteners attached to the inside of the box. What's keeping this box in place?
That is an older house and it appears that you have an old style wood and plaster wall. Sorry to say that the box could be secured in many different ways but here is something that might help.
The box you are going to use is larger than the original. Especially if you are using standard PVC boxes. So you can use the new box as a template to mark around the old box. Then remove some of the material around the old box which allows you to see how it is anchored in the wall and just maybe you can see how to remove it. I would use something like a multi tool with an osculating blade that moves back and forth.
If you don't mind a larger hole you can actually cut and install a duplex box and make it a quad receptacle instead of a duplex.
Hope this helps.