Swapping out the barrel, gas system and BCG when you want to shoot either calibre isn't easily doable... It's not an XCR after all!
That said, swapping complete uppers on an AR is as simple as pushing two pins out then in... You could easily run one lower and two uppers if you want.
I suggest you have two complete rifles though... The x39 primers can be hard, and you'll likely need a heavy hammer spring to make it 100% reliable. That makes your trigger heavier, and less than ideal for accuracy with 5.56. Maybe I'm just too picky because I compete...
The 7.62 x 39 round is inherently less accurate than 5.56, especially when using the cheapest surplus you can find (which is the whole point, n'est ce pas?)... It is still pretty good though, and will hit a 6 inch plate at 200 if you're prone or at the bench. Earlier 7.62 x 39 ARs were fraught with problems... The angle of the shoulder of the round made redesigning mags a challenge, but they seem to have it worked out now. FWIK the ASC mags are supposed to be the best.
The bolts can still be a weak spot. They needed to fit a larger base into the space of a smaller bolt, so they can break. It doesn't happen often, and if you get a quality rifle with a warranty (like NEA or Windham) they'll replace it. The LMT bolt is supposed to be the cat's pyjamas, but it's another $250-300 if you can find it.
Using corrosive ammo in a DI rifle requires a little immediate effort to clean the rifle and ensure no corrosion. There is a good video on YouTube going over his method here:
https://youtu.be/s8u5gUem55U
Questar sells the CMMG Mutant that overcomes all these issues by using AK mags and a completely redesigned bolt that is robust. That said, they're $2200, and there are no pistol AK mags available (unlike the 7.63 x 39 XCR pistol mags you can run in an AR).
Here is what I bought from a sponsor site: Great US made rifle and at 2014 prices.
http://www.kellysonline.ca/products/windham-weaponry-ar15-7-62x39-r16m4ftt-762
Bottom line: its a great way to save money plinking with an AR platform, but I wouldn't trust my life to it!