Ok, you need a new barrel, action group, lower.... So all you're keeping is the stripped upper...again, what are you "converting" don't see the benefit.
Ditto. All of the hype about the "quick caliber change" for the ACR made no sense to me. OK, so you can swap out the barrel, BCG, and magazine, and be shooting 6.8 or whatever else in seconds. That's nothing the AR-15 can't do already. With the ACR, you swap those things out, and now it's time to re-zero the optic (the one mounted to the top rail). With the AR platform, you just leave the optic on the upper as you change them out.
The ACR looks like a neat rifle and one that would be quite fun to shoot, but I've never understood why they made that much about the changing of calibers- it's more cumbersome than the rifle the ACR is supposedly intended to replace.