From my understanding (And this all needs to be taken with a pinch of salt), the modem has to "negotiate" with the CMTS a whole bunch of things. Part of this negotiation is establishing a downstream connection first, then the upload. For each of those, the modem has to negotiate things like what's the latest version of DOCSIS supported, what channels to use, symbol rate, QAM, all that good stuff.
Some of it's dependant on what both ends support, some is dependant on the quality of the line and some is simply configuration.
Igni is right about how DOCSIS3.1 does channels very differently. On 3.0 today, we have chunky channels that are like 8mhz each and bonded together. But I believe those channels aren't exactly right next to each other in the spectrum, you have to leave a "gap" or they could interfere.
On the other side, 3.1 has
loads of smaller "Sub-channels" (25-50
khz) that means you can make much more efficient use of the spectrum, because you can pack them in quite tightly:
Another bonus to this is they can increase the QAM rate, from 256 today to at least 4096. That's a fancy way of saying you get 50% more bits in the same frequency amount - on top of the gains of more efficient spectrum usage. However with higher QAM, you need less interference on the line. The improvements to 3.1 should mean it's better overall but I guess if you've got noise, you could drop down below 4096 and maybe even end up on 3.0 channels.
There's other improvements as well, like how error correction is handled and how the system can cope with bad lines. All in all, it's about making more efficient use of what's currently there.
We don't get the full 3.1 benefit overnight, though. You can't use the same frequencies for 3.1 OFDM channels as the older 3.0 channels, so they have to live side-by-side for a while:
Until eventually you get this:
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