Yes, the conversion can be made without opening the wall and it is a whole lot easier to do if the backing plate has openings above and below the main 1-5/8" diameter intake hole. If there are not openings you'll have to create them or remove the backing plate and replace it with this one (which is not easy) https://builtinvacuum.com/retail/shopretail.php?part_no=517
. The molded end of the romex in the rough in kit will go through the hole and into a new VacuValve faceplate. The faceplate has two screws, top and bottom, four inches apart, to fasten to the backing plate. The low voltage wire will go through the opposite opening on the backing plate and attach to the new faceplate just like the old faceplate. The non-molded end of the romex will have to terminate in a j-box or switch to get the two amps needed to power the electric power brush. When you make the conversion you can use any of our electric vacuum kits with direct connect hoses. Let us know if you have more questions.