Hello,

I am.looking at 650b wheelsets. I note.that most state compatibility with 10/11/12 speed cassetes

My bike currently has a 9 speed Sora cassete fitted

Will I just need suitable spacers to make this work?

Free wheels have not changed my pro 2 hope hub has had casettes from 9 to 12 on it.

Road and mtb casettes (not sure about 9.speed.though) are different widths though road are 2 mm ish wider so not sure in this case.

Yes. Road 11-12s freehubs are a little wider than MTB 9-10-11s freehubs. If you have a Road 11-12s compatible freehub you can use any trad spline MTB or road cassette, a 9-10-11s MTB or a 9-10s road cassette just needs a 1.85mm spacer behind it to take up the space.

(edit - that 1.85mm spec is for Shimano freehubs, other brands might be different but basically same principle)