Possibly the simplest solution is to acquire a 6.8 SPC upper receiver assembly, complete. Otherwise, yes, a bolt and barrel change will suffice. I would be inconvenient (really, impractical) to change back and forth from one caliber to the other.
You should also invest in magazines designed for the 6.8. Regular magazine function with 6.8 is hit-and-miss, mostly miss, or so I have been led to understand.
My son bought a stripped lower and an upper to build a 6.8. He ordered a 6.8 BBL and the parts he wanted just before it went crazy. It has been very hard to get any parts. The BBL and parts have trickled in over the last few months. He now needs the carrier/bolt.
Thanks for the info on the magazine, I will pass it on.
I also run the 6.8spc.....been shooting it for several years.....I started out with an upper and then finally built up a dedicated lower......I use the regular 6.8 mag's.....PRI's have been the best of breed, but other brands work well also.....I know several guy's who have harvested deer with theirs.....