Yeah I think I've heard that 10spd hubs don't do 9spd cassettes as well, but could you swap the freehub completely?

On a related issue, I'm running 9speed dura ace wheels on the hack bike, got em cheap secondhand, they're well used but very light wheels and they've always been silent when freewheeling. Just recently they've not been taking up all the time i.e. you might stand on it from a standstill or coasting and get 250rpm of zero load. It's quite exciting
I get the feeling they've had some of the drive pawls removed (for weight?) and the question is do I attempt my first freehub overhaul- clean/lube or assume it's buggered and get a new freehub?