What type of lineup change did you have in mind? Unless tragedy struck (heaven forbid), I'd imagine the band would go on hiatus or retire before any lineup changes were considered. They are pretty much in the twilight of their career.
Look, after John Bonham, Roger Waters, Mike Portnoy, Ric Ocasek... there is no one that is irreplaceable. The energy and excitement I saw in John Petrucci two nights last week, I find it hard to believe if someone bails that he's going to hang it up, but I can't really see him embracing a solo career either.
I have to agree with Stadler here. As I previously said, it's easy when all is good and nice to assume "oh, if someone goes, it's the end, we hang it up", but when the moment comes, when truly someone leaves for whatever reason, is any of them ready to give up what they're doing for a living? at a certain point it's not only a job that brings money, it's a way of life. It's hard to give it up because someone, no matter how much important, gives it up.
Look at Judas Priest, they're without "original" guitarists, they're old, isn't that enough? no, they just can't let go because that's all they know to do.
Also, I think nobody in the bands wants to go down in history as the one who killed it. Do you think, random example, Jordan Rudess would be happy to have Dream Theater disband because he suddenly decides he's given enough and wants to retire?