With just John Petrucci, no. He is the main songwriter, for both music and lyrics, but part of what defines the sound of Dream Theater is the voice of James LaBrie. While they did go through other singers before JLB joined the band, the simple fact of the matter is that very few people know, and even less actually like, the sound of WDADU and the Majesty Demos. For all intents and purposes, James is the only singer Dream Theater has ever had that most fans have heard, and he is therefore the only voice of DT that matters (and I don't say this lightly; I love WDADU and the Majesty Demos, but the reality is very few others share my feelings concerning those early recordingts). With JLB being the singer on every DT release from the past 23 years, to attempt to bring in another singer at this point (or later) would be tantamount to career suicide.
Now beyond that, I think the rest of the band members could probably be changed without too much of an upset. As difficult as it is to imagine Dream Theater without John Myung, if they absolutely needed to, I'm certain DT could find another bass player of comparable talent. The real question for me is that, with John and John having been in bands together since they were 12, would Petrucci want to be in a Dream Theater that did not include John Myung.
As for Jordan, while he is a big contributor in composing the music of DT, there were two keyboardists before him on four albums, with one of those records still bearing the honor of being the band's most commercially successful album, and another being held in very high esteem by much of the fanbase. Whenever Jordan decides to retire, if the other members of DT wish to continue on as a band, I'm sure they could fine someone else to take over on the keys. Perhaps Derek would reunite with the band, or it could be someone new entirely (I'm not holding my breath for a reunion with Moore), but they could certainly carry on without Jordan.
Lastly, it goes without saying that they could replace Mike Mangini if they absolutely needed to. No offence to MM, of course, but while he is of the highest class of virtuoso talent, if needed, the band could conceivably find another drummer to fill the position. Much like the keyboardists, I'm not sure if they would reunite with Portnoy, or if they would just bring in a completely new member, but if they ever needed to change drummers again, they could certainly do so.