Can a 89 231ax4 be converted to a 94 ax 15

I have done some research but can't get a definitive answer. Is it as simple as changing the input shaftsfrom21 to 23 or is even possible?

Changing the input shaft from a 21 to a 23 spline is just one aspect of the swap. You'll need the clutch pedal, narrower brake pedal, clutch hydraulic system, slave cylinder, and the clutch assembly itself. You'll also need crank pilot bearing 53009180AB to mate the crank with the AX15 input shaft.
It's certainly possible to do this swap and it's easier to do than the reverse AX15 to AW4 swap.