Is it true that pretty much all of the current grip frames can be swapped to any of the current single action frames - Single Six, NMBH, New Flat top, Old Army, etc.?
Yup. Only issue is the hight of the rear ears matching to the frame. Might be a verrry slight non-perfect fit. Ok, might have an issue if you swap a gripframe with a locking mechanism with one that doesn't have it. in that case a new hammer strut and seat will be required too.
I just swapped an XR3-RED grip frame to a 50th Ann. Flattop 351 and had to file the tabs off of the grip frame as there was no recess on the frame of the 357. There was a little difference in the height of the ears on the grip and the back of the frame, but no worse than my new factory NR-9L.