When I bought my internet plan there was no rental fee for the modem, can I use the modem as my own equipment on another xfinity account?

My current plan of 1200mbps goes for $70 with auto pay paperless and the 12 month introductory rate. When I got this plan almost a year ago, we were sent a modem and I set it up myself no problem. Now I'm looking to change to a new internet plan and there is an option to use my own equipment with the xfinity modem. As im not renting this equipment and own it, can I use it with my new internet plan as my own equipment? When selecting the "use own equipment" option there is an option for the xfinity modem but it only gives you the option to purchase the rental, not use one you already own? No offense, I'm not trying to pay an extra 15$ a month to rent equipment I already own since that seems a bit criminal. Help fast or being able to talk to a human rep would be appreciated.