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When you say "without changing anything" I assume you mean exclusive of the barrel. Aside from finding a 6.8 barrel, I understand the barrels are pinned in which means replacing it causes a major problem in itself. I also believe the mags are slightly different although the 223s could probably be tweaked to work. Not sure about the bolt head itself as I forget if the 223 & 6.8 head dimensions are the same.
Overall I'd say it's going to be a big (read expensive) job.