Now that we're at it, I've got a 5 sp MTB with 126mm spacing. If I go to 7 sp will it require a redish?
I'm not sure how MTB hubs were set up, I'm a road guy. I'd expect there isn't extra room, so if so the 7 will require more cogset or freewheel width than the 5 did. In that case, yes, you'll need the new axle, redish, and possibly cold-set the frame.