First off welcome to the forum.
I'm sure there are good gunsmiths who would do this, but I'd advise against it. If you spend much time here you'll see the 624 is a very sought after revolver. This conversion would make it less desirable to a collector. They didn't make many of them, and you're fortunate to have found one. I have a 4" model and it's one of my favorite revolvers. I would keep it as is and look for another 3" gun. Then you'd have two of 'em!
All the best,
Jim