I have a Colt Match Target 20" H-Bar that I'm thinking about turning into a M-4 type AR by changing out the stock and barrel.  Is there any difference in lowers  (other than the words "Match Target" stamped on it)  that would make this a bad idea?

Just buy a seperate upper and you should be good to go.   you can keep the origianl upper for future sales, it is better to have the orginal when you try to sell off a gun.

Seperate barreled upper are most likely have the small mil-spec pivot pin hole.  you might need a offsit pin to get the whole package working.

other then that, your lower is compatible with any M16 style uppers.

Thanks for your reply.  
Is there a difference between the Match Target lower and any other AR lower?
Different trigger or other internal part?

No, all lowers are the same (provided they are in 5.56 and not 7.62). That's the beauty of the AR, you can slap any lower on any upper and it will work. Certain companies (JP Enterprises, Socom Mfg., DPMS) may have uppers and lowers that look a bit different but they are all interchangable. If your lower  says Match Target on it, all it means is that it came off a Match Target Colt, nothing more. I don't believe the Match Target comes with a two stage or tuned trigger (I could be wrong), but I'm sure someone here will let you know sooner or later.

You probably have a large front pivot pin (non-standard).

The Colt will also come with .170" trigger pins compared to the industry standard .154"

You won't have to worry about the trigger pins if you're not changing it out, but you will either have to find a large hole upper or get an adapter for it.

WIZZO

All .223 lower receivers are essentially the same as far as upper receivers attaching to them, with ONE exception. Colt made some rifles years ago with a larger than normal pivot pin (.315"). Adapter pins are needed to mix and match large/small hole uppers and lowers. If your pivot pin is a two-headed screw, then you have a large pivot. Mil-spec is both the pivot and takedown pins being the same size (.250"). If I were a gambling man, I'd say that you have a mil-spec small pivot, as your rifle is a later model.