I'm used to assembling FALs, and headspacing... but not sure if that applies to AR's?  Can I swap my AR bolt for another AR bolt without worrying about headspace?

Thanks

If you have the guages, it's worth using them. However, I have swapped many a bolt in an AR, checking "almost" every time, and they have always checked and functioned fine.
I would say as long as neither the barrel (more correctly chamber) nor the bolts in question are extremely used, you should be fine. Most civilian AR's don't ever get nearly enough use to wear out the chamber or bolt.