I've done it
And it worked just fine. Actually, I'm MUCH happier with the downtube shifters. My mechanic gave me a set of shifters, and I just had to take it apart, clamp it on, re-adjust the railleurs and cut the cables- VERY simple. The only trouble I had was if I placed the clamp right against the cable stop, there was too much friction so I moved it like half an inch forward and its fine.
Good luck!