Slappy, I’m intrigued by the idea of switching free hubs. I need to explore that. Does it always require redishing? I need to disassemble a wheel and really see the parts laid out. I’ve got a bigger free hub mounted on a parts bent wheel. Think I’ll rip that apart and see what we got.
In short you have to do something, so yes...
Long story....
The higher capacity freehub (and sprocket) is "wider" in "steps", 6 and 7 are the same AFAIK, 8,9,10 speed freehubs are wider than 6,7 but all three freehubs are usually the same (there is some odd exceptions for them being the same--rare 10 speed specific freehub). 11+ speed is wider yet.
For the cassettes (not the freehub), a 10 speed is actually narrower than a 8 or 9 speed, it uses the same freehub body but with a spacer. 8,9,10 can of course go on an 11 with spacers. 6,7 cassette can go on the wider freehubs with spacers... but our question here is going the other way.
There are some machining options/tricks to get an 11 speed cassette on the slightly narrower 8,9,10 freehub. I had a thought that maybe I could do the same to get a 10 speed cassette (not 8 or 9) on a 7 speed freehub (remember the 10 speed cassette is slightly narrower than 9 speed) but it just won't work for me.
The wider freehub body (say swapping to a 8,9,10 replacing a 6,7) will add space on the DS shifting the rim and tire towards the NDS. The rim will no longer be centred. To keep the same drop-out spacing spacers need to be removed from the axle on the NDS. In some cases a mm or so of spacer may also need to be added to the DS if the new freehub has the bearing cup in a slightly different location (inboard).
But there are two options:
-Vintage steel frame. The rear hub spacing can be changed by cold setting the frame (within limits). In this case, lets say the new freehub shifts the rim 4mm towards the NDS. The frame HAD 130mm dropout spacing (say 90s steel). Replace the axle with the next wider size for 135mm MTB dropouts, cold set the frame to 135mm, check alignment of dropouts, hanger and the frame overall of course... Adjust the spacers on the DS and NDS to centre the rim and widen the set-up to 135mm. Cold set is no go for aluminum, CF etc. and if it is a "later" MTB it may already have 135mm spacing.
-Redish the wheel buy adjusting the spokes DS and NDS to shift the rim back to centre. Not all that hard to do but it falls between wheel truing skills and wheel building skills and may not be for all. Truing stand is a big help and a spoke tension gauge is also a really good idea to do it right, dishing tool will also help measure the centre but can be done without one. Basically slightly loosen all NDS spokes (roughly same amount) then tighten all DS spokes (same amount), measure, rinse and repeat. Follow proper building including spoke tension, etc. It will be different wheel to wheel and is sometimes worse with single wall rims but you have to watch that spokes don't push out past the ****** or bottom out in the ****** (or under-engage the ******), technically the DS is getting shorter and the NDS longer, but it is not a huge amount in effective spoke length.