So from what it sounds like is they are upgrading equipment to use more bonded channels. Cable modems have channels that are like lanes on a highway, used to be one lane each way. Then they went to multiple lanes to transmit so much bandwidth.
24x8 for instance is 24 bonded channels (lanes on one side of the highway down) and 8 bonded channels for upload bandwidth.
From posts above, I see that your surfboard is 4x4 and Comcast is probably upgrading to something like 24x8 or 16x4.
The CM600 or CM700 should work fine.
If you can tell us what your internet speed package is, like how fast your download speed and upload speed is, we can tell you better.
It should be a number like 200 mbps download speed, 10 mbps upload speed. Or 1000 mbps download speed (this is gigabit and really fast) or anything in between.
Also, can you ask Comcast what the requirements for the modem is? You can ask this question directly, if they can't answer it ask to speak to someone that can. Maybe try online chat for support if they offer it.
Ask this:
Do you need DOCSIS 3.0 or DOCSIS 3.1? How many channels upstream and downstream?
DOCSIS is just a cable modem standard, and some service providers are going to DOCSIS 3.1 while others are skipping it, Comcast should be able to tell you which standard you need, that will help you avoid wasting money on equipment you don't need.
Don't worry you're more competent than most "help this old woman understand" types of people. The service providers usually don't know what they're telling you to buy, and confusion = dollars to them.
