It is possible to modify a square butt to round butt configuration but it is expensive and you will never recover the cost in a trade or out right sale. I say that after having several N-frames round-butted. The following examples are still in the safe.
Top picture is my 45 ACP Mountain Gun in blue steel, a version S&W never offered. Below that is a 27-2 that was re-built, round-butted then bead blasted and blued.
Dave