I thought Cylinder & Slide used to do it as well as another, but checked their websites and saw no specific mention of this type of conversion. You might just have to call them. I have kicked this around for a while also, but with a 442/642.

I bought a new 940 back in the early 90s as part of my quest to find the perfect back-up. I was carrying a 9mm pistol at the time and thought that if my BUG also took my duty ammo it would be good. I recall the 940 had the heavy barrel profile while the 640s were tapered barrel. I don't know if this makes much difference. I got rid of my 940 after it locked up on me at the range. I had not fired a lot of rounds thru it, but had been to the range with it once or twice before and had experienced no problems. I had shot two or three cylinders of my +P+ duty round, and it worked fine. Then I tried some 147 grain Hydra Shoks that I had, and the thing locked up. I could not get the cylinder open for a while, and am guessing it opened after it cooled down. I could not see any reason for that happening at the time and did not think it seemed excessively hot. I took it back to the shop where I had made the purchase and they sent it back to S&W. When the gun was returned, S&W had not provided any paper documenting what work had been performed, if any and the shop could not tell me. I traded it for a .38 at that point. Hind sight is that I should have shot it again to see if it worked, especially considering what prices they're now commanding. For concealed carry and BUG, it was a great idea, but I did not care for the moon clips. They just seemed like they could get bend up trying to carry them in my pocket. I don't know if there were problems with the 547's extraction system, but if S&W had made that same feature available for the 940, it would have been great IMO.