When/how preparation for third-party modem?

I got a notice that my xfinity rented device is old and it can be upgraded for free. Instead, I want to buy my own third-party approved mode. I found the page that says which modems are approved. Can I just do this myself anytime, and send my device back whenever? or do I need to time this out, call in to schedule a switch, or call in to activate the new modem? Why is it impossible to find a phone number to call and ask these questions? Why is it impossible to get the xfinity chat assistant to let me talk to a real person?