Hmmmm. Interesting question. While it is a whole different critter, I was present when a barrel from a 9mm Beretta 92 was dropped into a Beretta 96, replacing the .40 S&W barrel. Same slide, spring, extractor ejector and even the same magazine, and it worked like a champ! Shot to the exact same POA, functioned flawlessly and even dropped the brass in a nice little pile just a foot or so to the right (where it flung the .40 brass half way across the county).
This is in no way intended to say that your suggested similar barrel drop in would work, just a little anecdotal evidence that it may not be as unlikely as some think. The only way to know for sure is to try it, and I for one wouldn't want to drop a considerable amount of $$$ on the chance that it might. Though come to think of it, I did recently buy an H&K P2000SK in 9mm hoping that P2000 .40 S&W mags would work in it (I have a very good supply of the .40 mags!). Fortunately, they work perfectly, and so do all the holsters I obtained for the .40 P2000.
One factor that makes it less likely to work - the 10mm spring would undoubtedly be stiffer than the .40's was, and may make it impossible for a 9mm to cycle, so you would probably have to swap the spring as well.
All this is assuming that a 9mm barrel would even fit the 10mm slide properly. They may be very different dimensionally.