If all the parts are there(and that can be a very big if) it shouldn't be any worse than taking off your LT fairing and then putting it back on. Heck, there are at least a couple of K75 owners who put RS fairings on their bikes. You might want to double check that the tanks are the same since the inner panels need to fit around the tank properly. One of the conversions was to a K75S, and the owner had to get a different tank.
My RS came with the "middle" bars, and the cables were all a bit taut. I think the clutch, choke and throttle cables are about 10cm shorter for the RS than for the LT. I am pretty sure that you don't have to change the bars. in fact, there are several threads where I have read of owners putting RT/LT bars on an RS so they don't have to lean forward so much to save their backs.
I would spend some time with the MAX BMW parts fiches to get an idea of all the parts that go into the fairing. One thing I have learned is that the 1100 has different knee pads and side covers from the older 100 so you might need 100 side covers to make everything work. I was able to put the 1100RS fairing lowers and belly pan on a K100RS with a minimum of hassle so I know there is a fair bit of interchangeability between the 100 and the 1100.