Are you sure the guy wasn’t trying to make an H22a without cutting the hopper down?
The actual request was to make extension that fit the N scale H21 to look like the H22 Bowser made in HO. I think you are correct about cutting the sides (or going to town with a belt sander) and adding a new top rail; which could still be done I suppose if you want it to be more accurate.
Now you've got me thinking-- If the height of the H21A is the same as the H22A, the model could be made by adding the "fake" original top rail around the H21 mode, but you'd need to also sand off the ribs. A kit with 4 pieces with slots for ribs, and maybe the top brace. Seems like a lot of work.