I have a Shogun 12 speed, and removed the tall gear at the front and left 42t at the crank and gonna work on the rear wheels. This bike was made in the late 80s so it utilizes the freewheel hub. If I were to get a 16t at the rear, I would have to respace rear axle and redish wheels if I reuse my rims that were meant to carry those 6 gears. Now should I go out and buy new rear wheel? or respace rear axle and redish wheels?
I didn't have to respace and redish my wheel when I did a crappy 27" conversion; my chainline came out to be very straight with my smaller chainring up front and the wheel was perfectly centered. Maybe I was lucky, I don't know.
Redishing could snap spokes if they are old and corroded/rusted and it'll be a pain if you snap quite a few of them. You could get 5 new spokes of the same length on both side beforehand. I wouldn't get right into redishing if your wheel is not perfectly centered in your dropouts and your chainline is not a total mess.
see, I don't have the granny gear, my bike is so old it has the "middle gear" and tall gear. I'm using the middle gear and 16t, but thinking of just buying new wheel set since that was my plan. But it would be nice to see how it feels first.