Friend of mine did it with an old Centurion with mixed results. He'd had the bike for 25 years, did many centuries and thousands of shorter rides on it and didn't want to give it up. The bike had a one-inch threaded steerer, so the first hurdle was to find a one-inch threadless headset or a carbon fork that would work with his old headset. I can't remember which he finally did, but it took awhile to work it out, and when he did, he wasn't impressed. It worked fine, but it cost quite a bit (this was back when carbon was new and expensive) and wasn't a big improvement on what he had. Then about two weeks later the bike flew off his hitch-mount rack and was totaled (his fault--he hung it on the back, changed clothes and drove away without securing it). He bought a Litespeed.