Effect of replacing modem/router on an existing extender

My current modem/router is the C3700 Docsis 2.0. I just got off the phone with Comcast, and they informed me that I need to upgrade the modem to Docsis 3.0. I'm considering purchasing the C6300. However, a year ago I purchased an extender, the EX7000, and I'm not sure whether I'm going to encounter any problems with linking the EX7000 to the C6300. In fact, I don't even know what the steps are to link the extender to the new modem/router even if I can do so. Can anyone help? (Sorry if my question sounds like I'm clueless, but I'm not particularly tech-savvy.) Thanks.