Look, one thing that (I think) you can do, if your smallest (thread-on) cog is OK, is get a HG cassette, take it apart, and reduce the size of the largest projecting tab on each of the cogs with a Dremel grinder. I think that Sheldon Brown mentions this on his freehub page. Then the HG cogs will fit on the UG freehub body. OTOH, if your freehub body has threads on both the outside and the inside of the outer lip -- some do, can't see yours -- then you can also use the smallest cog from the new HG cassette, and thread it onto the interior threads.