Upgrading from XB6 to XB7

Hello. I've recently updated my current plan from 600mbps to 1.2gpbs. When I was signing up for the plan online, it denoted that I'd need to return my current XB6 equipment in exchange for a new modem. However, the website gave me an error that needed to be resolved with support. I worked with online support to get my plan started but they stated that I didn't need any equipment change. I've been testing speeds on my local wireless devices as well as reviewing user feedback, and I believe I'd need to switch my XB6 to XB7 in order to fully utilize the upgraded speed.

Can you kindly confirm this and point me to how I can exchange to an XB7 to fully utilize my new plan? Can you also confirm if there are any costs associated with exchanging? I currently rent the XB6 at no cost.