If I want to add an additional upper for my rifle can I use my existing bolt carrier group with out a high chance of headspace trouble?

Well, that depends. Do you feel lucky?

First, everything in the carrier EXCEPT the bolt itself is interchangable with no safety issues. And the odds are good that the bolt will work OK in the new upper; if they are the same make and both fairly new, the odds improve.

Still, we're talking odds here, not certainties. So whether it's a matter of 1 in 100 or 1 in 10,000, there is still a chance you'll create an excessive headspace situation.

So, weigh the cost of a new stripped bolt to match your new upper against the cost of even a minor trip to the emergency room.

Some say you can do it, and some say you can't. I would suggest getting a complete carrier assembly to go with your new upper. You know sooner or later you are going to get another lower to go with it.

The price of a headspace gauge (the 'field' gauge) and a few factory loaded rounds isn't much, and if used properly, you can be sure with a high degree of certainty if a given bolt headspaces properly to a given chamber.  It's definitely worth it to get the field gauge, and learn its proper application, which is simple:

The bolt should not close fully on a field gauge, but should close completely on a round that's to factory specs.   And of course, the lugs must cam to the locked position.

When checking headspace, remove the upper from the lower completely.  You're not going to ram the gauge into the chamber with the power of the recoil spring, you're going to push the bolt carrier into position by hand, with your thumb and no more.

Investing in the other headspace gauges, the go and no go gauges, would be worth it if you're going to build or work on more than a few AR's or other rifles, but for just a couple of rifles, you need only the field gauge and some ammo that's physically to spec, which will be any quality factory ammo.

CJ

I thought about swapping 1 bolt/bolt carrier between 2 uppers, until I saw in the Equipment Exchange that they were going for about 100 bucks shipped (Bushy wants 120+shipping) so I figured, what the heck.

Not meaning to ignite or counter what anybody else has said here, it's all sound advice, but I have a personal experience that might be interesting.  Remember take all information from Gore's highway with as much faith as if it came from Gore himself.  I assume no responsibility for any lost fingers eyes or blood. Batteries not included.

When in the Army after a day at the range we would come back to the barracks to clean rifles.  The squad I was in would do kind of like an assembly line cleaning where one fellow would scrub bores with a bronze brush, another would wipe them clean, somebody else would run hot, soapy water in the lowers and scrub them out with a tooth brush and Q-Tips...I would take the bolts and bolt carriers down to the latrine and disassemble them, including the bolt, extractor, ejector, and throw them into a sink with hot soapy water and go at them with a tooth brush.  There were probably 8 or so in my squad.  Hell, it would have been a miracle for any bolt to get it's original parts back, let alone get back into the right carrier or rifle.

After a couple of these go arounds everybody in the barracks was cleaning their rifles this way.  

Never heard of a KaBoom.

That's what happens when your main motivation is avoiding an ass chewing from TOP because his white glove came out with a black streak on it.