If there are no internal threads on the end of the freehub, that's a very rare 8-9-10 speed UG 7402. Not many of them were made, and they will only accept a lock ring down to 13 teeth, because they have the same external thread diameter as other UG freehubs (the earlier 7401 and 7400 freehubs had a smaller lock ring thread diameter and coild take a 12 tooth). But check to see if there are internal threads on the end AND one smaller spline that will accept HG cogs.
Easiest way to put HG cogs on a UG freehub is to enlarge the one small notch on an HG cog with a file or Dremel, use the correct size spacers on the cassette, and then hold it down with a UG lock ring. Works fine.
Some of us use UG cogs and think they work just fine, but the switch takes maybe ten minutes. On the other hand, if you're convinced that HG is the way to go for you, then hold out for a set of HG wheels or hubs. Plenty of them out there, and I'm of the opinion that pretty much all of the Shimano 8-9-10 speed freehubs are equally excellent, from Tiagra and 105 thru Ultegra to DA.
__________________
Hard at work in the Secret Underground Laboratory...