Since no one else has replied I'll let ya see what I found at their website in their FAQ. Hope it helps:
Q. Will Bushmaster change barrels if someone sends in their upper receiver?
A. If you decide to (or need to) re-barrel your upper, our gunsmiths will install most any parts we sell with no charges for the gunsmithing services (we do charge for front sight base installation & removal).
hmmm, dunno how much front sight removal and install would cost but the smithing is free.