First you need to get the control lever that you want - Numrich has them, occasionally. If you are making any functional control changes (going from SA/DA/Decock to DA Only) you may need a different detent plate, too.
There are instructions on detail stripping a USP out there - it's pretty simple. Only one spring that I can recall that will really surprise you.
I will try to find links to parts / breakdowns later tonight.
/rl