Most bike hubs are 130 (road/hybrid bikes) or 135mm (mtbs) and the rear dropouts are similarly spec'd so no need to have the stays adjusted which btw would be possible on a steel frame, no aluminum. Cannondale? sounds like an aluminum frame, so no go.
Hubs can accommodate 8, 9, 10 and even 11 spd cassettes. The only difference is the numbers of cogs and spacing between them. An of course, you'll need the corresponding shifter matching the cassette. Also, you'll need the appropriate rear derailleur for the cassette. Sorry, I can't speak to your original 7spd hub. Never had a bike with less than 7 speeds in a looong time that I really don't know if 7spd hubs are spec 'ed differently.