Most 300 Blackout barrels have pistol gas ports, so you'd probably need a gas block and gas tube as well. And don't forget that the muzzle device will need to be a .30 caliber item - not only a .30 cal hole, but almost all the 300 Blackout barrels I've seen have a 5/8x24 (as opposed to the .223's 1/2x28).
Instead of saying "just the barrel needs to be changed," it would be better to say "the barrel assembly needs to be changed.
8" barrels are popular, as are 10.5s. I have an 8" CMMG barrel that's nitrided, and I'm just waiting for the chance to load up some rounds to try it out. CMMG and Aero Precision have decent prices on 300 Blackout barrels.