DOCSIS 2 vs. DOCSIS 3 differences ?

The main difference between DOCSIS 2 and DOCSIS 3 standard cable modems is that DOCSIS 3 allows for a much higher maximum throughput. DOCSIS 3.0 achieves that by using multi-channel bonding (using multiple channels simultaneously for download/upload) while DOCSIS 2 modems do not. A Docsis 3 modem with 4 bonded channels, for example would provide 4 times the bandwidth of a Docsis 2 modem.

Some other technical differences:
Docsis 3.0 supports QAM128 for upstream traffic, while many DOCSIS 2 support only QAM64
Docsis 3.0 specs use 108Mhz to 1.002Ghz downstream, 5Mhz to 85Mhz upstream.
Docsis 2.0 specs use 88Mhz to 860Mhz downstream, 5Mhz to 42Mhz upstream.

Changing to a DOCSIS 3 capable modem isn't going to change your maximum speed up, down or
sideways, only subscribing to an upgraded tier will. However, DOCSIS 3 has the ability to use multiple downstream and upstream channels at the same time (channel bonding) to maintain the maximum possible speed. Because of this, DOCSIS 3 may experience less of a speed drop during peak usage hours vs. a DOCSIS 2 modem. The D3 modem, depending on how new it is and how many channels the ISP makes available, has access to 2x-8x more bandwidth than a D2 modem, allowing for a somewhat more consistent connection.