Heat the bung with heat from an oxy-acetylene or Map gas rig. Propane won't do it. Also using a 02-specific socket helps. I have never had one I couldn't get out with heat. Sometimes I have to remove the upper IC for access.
Do what ever you can to get it out. Changing the down pipe in one of these is no easy task. It's a bitch, and the car has to be pretty far off the ground. Or take the turbo off first and the DP will come out the top. Even if you cut the old one to get it out, you have to put the new one in.
This is the socket I have than never lets me down. Even if you have to drill it out, and chase the threads, it's easier than dealing with the down pipe. With the IC removed, access is pretty good.
And if you break off a stud or two in the turbo, you have a whole new set of hassles.
I know what I'm talking about. Get the old one out. Or weld a new bung in the old pipe. Anything other than changing the DP for this. There are much easier and cheaper ways around this.