well i swapped the freehub bodies, that was really incredibly easy actually. I didn't need any parts from the donor wheel besides the freehub body.
To my surprise, when I put the wheel back on with the 7speed freehub body, the wheel was perfectly centered in the frame, and didn't require any redishing...!? I guess the 6sp UG body i have is the same size as the 7sp HG..? it kinda makes sense since there is a some of the threaded UG body exposed.
This is pretty unexpected, but if I don't have to redish the wheel, I'm not going to bother
If i start noticing the chain making contact with the stay/drop out then i'll mess with the order of the spacers on the axle and redish the wheel. I gotta believe 10 speeds are running with tolerances like this though..