You can make HG cogs fit the UG freehub by grinding down the fattest tab on the cogs. You'd still need to use a threaded UG top cog as there are no threads for a HG lockring.
Replacing with HG freehub is also possible and is easy. I did it on my 105 UG hub. There could be issues using the cone nuts from your 105 hub with the HG freehub - i.e. some freehubs require special cone nuts. But it may also work fine. I actually bought a complete HG rear hub (HG-20 I think) and moved the freehub and drive side cone nut over to the 105 hub.
Last edited by Gonzo Bob; 07-31-08 at 04:01 PM.