There is no "old" or "new" Dream Theater. Well, A Dramatic Turn of Events is "new" Dream Theater.
To me, a good number of their Portnoy-era albums are in "light/dark" pairs, and that's about it. (I say "Portnoy-era" because this is what I know to definitely be true. To me. We don't know if they'll still do this with Mangini. Plus, it's like saying "Gabriel-era" and "Collins-era." :3)
When Day and Dream Unite is paired with Black Clouds & Silver Linings in that they aren't actually part of a pair, but they're bookends of the Portnoy-era.
Images & Words is a light album with heavy progression, and it's followed by Awake, which seems to follow similar progression but with much darker themes. These are the "mainstream pair" to me, because everyone loves these.
Falling into Infinity was a really good album, though not terribly progressive. It was intended to be progressive, though! Likewise, Metropolis, pt. 2: Scenes from a Memory, in my opinion, was a really progressive album but not that good, yet it was intended to be good. They balance each other out! And neither one of them sounds much like their surrounding albums. These are the "transitional pair."
Six Degrees of Inner Turbulence was a light album with, as one poster so elegantly put it, a nice Liquid Tension Experiment-esque groove. It also marked the start of what I call the "Octavarium cycle" (wherein the amount of songs goes 6, 7, 8, 7, 6). It was followed by the darker Train of Thought, which also contained the LTE-esque groove and.. well, I thought it was rather inseparable from Six Degrees. I call these "my favourite pair." Or "Six Degrees of Thought."
Octavarium was sorta like the past two albums, but it was clearly much more simple, with much more focus on concepts rather than extremely complex songwriting. It was also pretty light and serious, compared to Systematic Chaos, which was darker and less serious. Now, some of my favourite songs are on this pair, and after stating I'm not the biggest fan of "the mainstream pair" and Metropolis, pt. 2, I imagine a lot of people here are questioning how big of a Dream Theater fan I am. Well, I just love finding classics in modern music. >.> ...ANYWAY. I dub this pair "the black bar pair," for their album art both feature black bars.
Finally, we're left with the almost fridge-brilliant Black Clouds & Silver Linings, which "octavariums" its way back in time to pair up with the other odd one out When Day and Dream Unite. Black Clouds is also fridge-brilliant because it deals with the theme of "Every cloud has a silver lining," which is uncannily similar to my "light/dark" pairing of albums.
ERGO, I conclude that Metropolis, pt. 2: Scenes from a Memory is neither "old" nor "new." In fact, I conclude that none of Dream Theater's discography is "old" yet. Maybe in another decade will any of their music be considered "old." For now, their work is just music. If you want me to categorize Metropolis, pt. 2 as anything, I'd call it a transition, much like its predecessor Falling into Infinity was. For they were the "transitional pair."
I'll shut up now. :D