To be brutally honest, Mike Mangini is the only "expandable" one, not because there's anything wrong with him - but merely from a perspective point, the band existed 25 years without him, another drummer will just do.
Of course I don't want him to go and I'd be sad to see it happen, especially now that he's finally fully integrated in the songwriting process, I'm just speaking hypothetically here.
The other four would be a huge blow. With JP mostly but also JR, you lose the songwriting core of DT, and in Petrucci's case, essentially the heart of it all. Take Petrucci out of DT, and what remains of DT is just the name. Myung as another founding member would be a huge blow, maybe the songwriting could survive but Myung is essentially tied to DT.
And James... say what you will about his live performances, he's the defining and ultimate voice of DT. DT = James singing. It's not a matter of being able or not, probably there's out there someone who could sing all the tunes - but after all this time, no one else would feel right. It would not just sound like DT, because James' voice is forever tied to the sound. Freddie Mercury reincarnated wouldn't feel right singing DT, and certainly not any other singer of the scene or an unknown talent found in a cover band. "Nobody" accepted Ripper in Judas Priest after all those years, and the guy could scream like a muthafaker. But he was not Halford. No one else is Halford but Rob Halford. No one else is the voice of DT but James.