Is Comcast supplying you with the modem or are you expected to supply your own?
If I understand correctly, you currently have a separate modem and router.
Assuming they are separate, it's the modem that has to be DOCSIS 3.1 compatible and not the router -- and Comcast will be supplying you with the modem.
Data Over Cable Service Interface Specification
It has to do with the connection between the modem and your ISP. As regards compatibility, it doesn't affect the ethernet or wireless connection your computers have with your router, or the connection between your router and the modem.
The reason to upgrade the router is that with the old router, you're not getting the speeds you could be getting (assuming Comcast is actually giving you those faster speeds at your subscription level). Your ethernet speeds will be capped at 100 Mbps for ethernet with that E-1000 router -- so if that router is connected to the modem by ethernet cable, you'll get a maximum of 100 Mbps. The 300 Mbps wireless is an optimistic maximum speed, and in all likelihood you'll get a lot less.