Also, I'm getting kinda weary of reading all the posts saying DT's new drummer is going to lead them to a new renaissance or something. Drummers just don't have that much of an effect on the music. The majority of the music is written by Petrucci and Rudess. Mike was deeply involved too, but not in terms of writing. Mike was more like a producer, giving ideas about how songs should be arranged.
But in the role of "producer", MP appeared to have a lot of influence.
Eh. At the end of the day, he wasn't the one coming up with the riffs.
We only see a snifter of the process, and of that snifter we know that he wrote the main contingent of A Rite of Passage, for one.
He's a musician. He writes music. And arranges it. Admittedly, none of us can tell a Mike Portnoy riff just by listening. That means they're probably fairly indistinguishable from the JP riffs. They've spent twenty-five years writing together, too, so they'll have synched quite well. But it's very hard to write like somebody else. No matter how much King Crimson influence Maynard James Keenan has, his songs still sound Maynarddy. And Mike Portnoy's music sounds Mike Portnoy-y.
It's hard to tell
how it'll change. But you're removing one unique mind from an increasingly small hive. And a fairly pivotal one, too. It will sound noticeably different. It will still sound like Dream Theater. They won't become a jazz fusion band. But Petrucci's main collaborator will become Jordan Rudess. Mike Portnoy's idea of what belongs on the cutting room floor is different to what John Petrucci's might be. They won't transform, but the emphasis will shift to different places. Petrucci's quite melodically inclined, Rudess is more technological.
The sound absolutely will change. It won't be a revolution, and they'll be continuing along the same line, from where they left off, but it's actually impossible for a band without Mike Portnoy to sound identical to a band which has Mike Portnoy in it.