hi yes was hoping to go from sqaure to round but the wiring isnt the issue. it dont look like it will fit without making hole bigger that it goes into.
I bought some round ones and they had a backing plate that fixed to body work then the lens fits to the plate,just had to drill new holes for self tappers in body work to fit the backing plate.
They are commonly referred to as NAS (North American Spec) lights and people have been fitting them for years (did mine in 2000), plenty of information on the different makes and how they fit.
My personal preference is Le Perei, they were the original supplier to LR and their LED offerings are top quality and far nicer than the likes of BOB or Wipac.
If you use the normal bulb type then yes, you will need to drill the bodywork to take the rear of the lamp body but if you fit the LED type then they are flush backed and fit directly into the mounting plinth.