7 to 9 speed?

Sorry if this is in part slightly post-retro. Also, it's oft discussed but I seem to have got a bit lost in the details!

Just bought a '98/'99 RSP 250 (see viewtopic.php?f=4&t=243797), with which I am very happy - lovely frame. It rides very well as it is, but been down-spec'd - in particular from 27 to 21 speed. The shifters are nasty combo modern 3x7s - which have to GO. Now I don't mind 7 rear, but all my other wheels etc are 9 and I prefer interchangeabilty. I could just fit friction shifters, but I happen to have some index 9 speed to hand and a set of new red DX V levers, which would be good with the red decals!

I could junk the wheels, BUT the as new Mavic M231 rims seem to be worth keeping. The hubs are also nice nick polished DX - and thus (I assume?) do not take a 9 speed cassette.

Seems to me three options. Rebuild the rear on to a new 9 speed hub. Replace the DX freehub with a 9 speed (I THINK that this is possible - with a re-dish/re-space?). Remove one cog from a 9 speed cassette to fit on the existing freehub, setting stops accordingly and altering when I swop wheels. For the cassette chop I imagine easiest is to ditch the largest?

What have I missed? All comments (up to and including derision) gratefully received!

Myself, I'd just rebuild the rear wheel if you want true interchangeability between bikes.

I've done the free hub body swap on mine, works well

Thanks for views!

Having checked the DX/M231 wheels and fitted some fatter tyres they look so nice that I'm going for 7 speed thumbies with the friction option (which I always prefer for front mech anyway).

Good choice and your name proves it

Would that be the OLD(ave) or the (ol)DAVE bit?!

Would a 10 speed cassette with one sprocket no fitted work. I am sure I have seen a write up on on Sheldon "the Good" Brown's site somewhere. Never tried it myself.

I think TEN speed would only complicate matters - at the least all the other 9 speeders would need new chains!