ok, I caved, from the horse himself:
Freehub Body Swapping.
Use two 5mm alan wrenches to remove drive side (Cassette side) end cap. Insert a 5mm alan wrench in each side of the rear hub. Pull cassette side end cap in counter-clock wise direction.
Slide the existing freehub body off the axle. Make sure the spacer behind the freehub body stays on the axle. Make sure pawls come out with the freehub body. Sometimes the pawls will stick inside the hub shell. If this happens, use fingers to pull them out.
With a cloth, clean the internal ratchet ring area.
Use light grease or oil on back of freehub body, ratchet ring and on the spacer. Place some grease on axle.
Slide on the new freehub body. Make sure it slides all the way in and the pawls engage the ratchet ring. Reinstall the drive side end cap.
Switching from Shimano to Campy or Campy to Shimano.
Note: Older hubs may need to have the rim dished over a 2 to 3mm for proper dishing. New hubs, as of the fall of 2008 do not require dishing.