There are many threads on this over in the Google groups Surly LHT Forums.

http://groups.google.com/group/surlylht/topics

I can't say definitively how hard it is because I am still waiting on my Paul Canti Levers to get here.

It doesn't sound that hard to me. Although I plan to use Paul Thumbies, rather than twist shifters.

The one question I still have open is whether I am going to need to replace my brake cables when I replace the brake levers.