Can you change them or are they actually fixed into the disc itself? The reason I'm asking is because I have the chance of buying a screw on cluster type disc wheel, but I have a 9 speed bike (ultegra). They don't make 9 speed screw clusters!
Most hubs on discs or other solid formed wheels are fixed into the structure.
I asked about this very thing at my LBS as I had a flash tri-spoke rear wheel with a screw on 8 speed. I wanted to get in to work with my 9 speed ultegra as well - but to no avail.