...Further, if it had been converted from .30-'06 to .308 with a Navy sleeve (that "bushing" thing), you really can't "un-convert" it.
Although you probably can remove a Navy sleeve by carefully jamming a tapered round file into it, and twisting and pulling, I believe that you would find that the .30-'06 chamber is essentially ruined by the imprint of the jammed-in sleeve.
Although I have no personal experience, firing a so-converted Garand, I am led to believe by the comments of others who have done so that the combination is not as accurate as the original configuration. The culprit, I've been told, is the excessive length and oversize diameter of the sleeve's freebore, between case mouth and barrel rifling.
The magazine-and-clip system works just as well with one cartridge as with the other.