I overheard someone today at a show telling his friend that if he bought an upper and bolt seperately the gun would need to be headspaced.
Also that if he ever changed the bolt it would have to be headspaced again.Is this correct?

Wrong, the headspace is fixed. buy a bolt put it in and shoot but, if your worried you can always buy a headspace guage so you can sleep at night.Just buy good parts bushmaster,colt,RRA,Armalite and you should have no problem
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Not headspaced but checked. The bolt should be a matched set to the barrel/extension ring.

(I don't play the lottery either)

Slap in the new bolt, pull the trigger once and if it does not blow up your good to go lol.

Really though what FREFALLE6 said is accurate.

The chance is 99%+ that your bolt will headspace just fine.  You friend is correct in that each bolt will headspace differently, but the headspace is fixed when the barrel is put together with the extension, and that's something you aren't likely to be able to change.  I have yet to come across a bolt that would not headspace properly, and for one not to, it would have to be SERIOUSLY worn out, and I mean WORN, or incorrectly manufactured.  I check headspace just for fun since I have the gauges.  The Colt M16 field headspace gauge is extremely tolerant.

Just throw the bolt in and hit the range.


Edited to say:  The only time I would be concerned with headspace is when trying to find a bolt that would headspace really, really tight for serious benchrest accuracy.  My SPR bolt is fitted to my barrel (extension).