Originally Posted by
pstock
I might investigate whether I can swap the 7S body for an 8S body.
To what end? The three additional ratios above the twenty-one you already have will likely be redundant or nearly so. It seems like a lot of work for little gain. If you need higher or lower gears outside of the ones you have 7 speed cassettes are available with various large and small cog combinations (mountain vs road) so a cassette change may accomplish that. "Never Fix A Running Piece" is often a sound course.