This is a wierd thread. In my experience with a 1994 NP231 and a 1998 NP242, I can safely say the input gears ARE NOT interchangeable.
I found that I could not swap the input gears from a 231 to 242 and vise versa. They are different sizes.
However, as to the original point made by the initiator of this thread, yes you can swap the whole 231 with the whole 242 as long as the spline counts are the same. If the spline counts are different, Advance Adapters (or a junk yard) would have an input shaft (input gear, actually) from the SAME MODEL transfer case to get you the right spline count.