Re: how hard is to replace the nut of a les paul??
It depends on who is doing it If you have not done it before, I wouldn't suggest trying it yourself on this guitar. But as long as you have 1) the old nut 2) the proper tools (glue, files/cutting, etc) and 3) patience, it's not too hard at all.
Any decent tech can replace it very easily if you have the old nut, it might even be better since they will be starting from scratch and can make sure everything is aligned, spaced, and cut properly.