Gunsmithing question on cutting down a barrel

I bought a gas gun that was built using a mega monolithic upper/lower with a 24" Brux barrel. I want to cut the barrel down and re-thread it. Will the gunsmith have to remove the barrel from the receiver to cut it down and re-thread or is it done assembled?

What I am thinking is whoever works on it has to have a special mega maten wrench to remove the barrel if that is necessary.