While I haven't checked J frames, I have seen many K frame revolvers where the serial number is stamped such that you can not modify the square butt to a round butt. The serial number is often stamped toward one end or the other on the frame. A grip conversion requires that you remove metal from both sides of the grip frame. Before doing any work, fit a round butt grip on the square butt frame and confirm the serial number location will allow for the conversion. The screw for the main spring will also need to be changed or modified.
I have done this on two N-frames and one K-frame.
The trick is like stated above: You can't cut off the serial number. Just take a set of K-frame Magnas and trace them with a pencil on your frame. If it will not remove the serial number, no worries.
Take your time and do it slow. In all of my conversions I did not alter the front of the frame, because the grips I used after the cut covered the front strap, and it is FAR easier to modify a set of grips you intend to use (I my case Ahrends finger grove) than the frame.
It's a great way to make a square butt revolver a dedicated carry piece!
I did this on a 65. As stated previously, check the serial number location. I rounded the front of the butt as well and the strain screw did project out and should be replaced. However, as I put on Hogue grips, I let the screw squeeze into the grips. The most difficult part was maintaining the contour of the back of the butt where the metal was removed.
The square to round butt conversion will work on the "K","L", and "N" frames, but will not work on the square butt "J" frame. I own both a square and several round butt "J" frames, and have checked this myself, it's a no go.
I've never used their services, but they seem to have a solid grasp on weapon customization. They offer the round-to-square conversions for $95...although you'd probably pay more than that just for shipping back and forth!
I don't think I'd be too trusting of myself removing metal off of one of my beloved Smiths, so I'd look towards the "professional" route if I was looking conversion. Either way, good luck.