I'm rewiring my house, which was built in 1921. I need to bring new cable to a triple-wide switch box (see photos). I cannot push the old knob & tube wire out through the knockouts--it must be stapled near the box. There is an unused knockout, but I haven't been able to knock it out from within the box. I removed the mounting screws, but it is set firmly in the plaster, so I can't just slide the box out.
Is there a trick to pushing the old cables out, or removing the knockout from inside the box? Maybe drill a hole in the knockout and then pull it inward? Is there any way less destructive than cutting all around the box to remove it?