I've logged into my basic account (which I've had for a while) and don't see anywhere to enter my credit card information without agreeing to pay. Can anyone point me in the right direction?

Make sure you're logged in and click the above link. It brings you to an upgrade page.

Read the fine print, it says "charge me $100 year after October 6th" or "charge me $9.99 monthly after October 6th". I chose monthly because I don't want to get caught with a $100 charge if I forget to cancel.