Downgrading Internet

I’m currently renting equipment from Xfinity and my first year contract just ended so my bill has increased. I don’t need the tier I currently have. If I try to add a lower tier to my cart and checkout? What happens? Do I need to send my current modem back or is there a way to just continue renting the one I’m renting now? Do I still have internet access right away with a new plan or do I have to wait? I just want to lower my speed so I’m not paying for something I don’t even really need.