g'day Lew!
Although I have no personal experience with inverted forks, or USD's, I am led to believe that conventional forks are now considered to be 'better' than USD's for enduro/trail applications. I'm not too sure of the technical reasons for that, but you may have noticed some brands of bike that previously sported USD's are now back to conventionals. KDX was one of these, i think it was the '93 that had USD's, then later models went back to conventional.
I do know that my conventional forks don't blow fork seals as often as my mates WR200 with USD's...
Anyway, back to your question, it would be costly I would think, to put USD's on your bikes. you would need new upper & lower triple clamps for a start, and then your head stem bearings could end up being the wrong size; you'd need new brake calipers etc etc...
Far easierto put in an aftermarket set of springs, and perhaps some revalving with a reputable suspension specialist. I can recommend XR400 springs of a suitable rate for your weight, as that is what I have put in mine...
Anyway, hope this helps you a bit,
Cheers