[villagetinker]

Wow, I would expect this to be expensive. The wires are in plastic conduit in the concrete slab. At a minimum I would expect a concrete saw (aka LOTs of dust) the cut a new trench for the new conduit and hole for the new box. NOTE: the existing box cannot be removed as there would be a junction there, so at a minimum you would need to have a removable cover plate over the junction box.
We were able to locate flat extension cords that allowed the existing box the be covered with furniture and not interfere with the cords.
A licensed electrician may have other options, but I expect this will be s dirty and expensive process.
These are just my thoughts, and I hope they help. Good luck with your project.