I know John mentioned in 2011 that the current line up will not change. So that's that then.
Got link?
yes: https://www.youtube.com/watch?v=0su0n0CBXsg
It's been AGES since I've seen that clip. But, yeah, it's about how I remembered it, and how others have described. To paraphrase, it's essentially: "Mike Mangini is a
permanent member of DT, and we have no intention of that ever changing. But you never know what's going to happen in the future."
And in line with that last line, JP even says that if you asked him whether the previous lineup with Portnoy was the permanent lineup in perpetuity, that he would have said yes, obviously not knowing that things were going to change and Portnoy was going to leave.
I think the important thing to keep in mind is that, as I, and Kev, and some others have said, they have expressed that, when they brought in Mangini, their intent was to have him join as a full member and not a temporary hired gun. They had, and do not have, any intention of looking for a replacement. But that doesn't mean that things don't change. He
could decide to leave on his own. Or he
could be asked to step down. And if that happened and the band wanted to continue, they would consider replacements. Depending on how everyone involved feels, that could be Portnoy. Or it could be someone else. And the result could likely look very different depending on the timing and circumstances, were Mangini ever to exit the band for any reason.
EDIT: Or what PG and Stadler said--although my post was much more eloquent than theirs, so you can feel free to disregard theirs and read mine only, and just "+1" it twice.