Here's an update on the Gypcrete. It was barely 1" thick and poured over a 3/4" chipboard subfloor supported by gluelams every 12 inches. It was extremely easy to cut, I cut it with my recessed can hole saw. You could carve it out with a screwdriver if you wanted. I was able to remove the Romex from the unwanted box and fish it to the new box, which I supported by fastening a 2x4 platform between 2 gluelams. First I fastened two pieces flat against the gluelams then placed the platform across them.
I cut the old box off flush, then filled it with a gypsum based concrete patch, patched my holes and filled in around the box. Job done and fire rating intact.
It was way easier than trenching a slab.
