How to extend an Office 365 license with a new product key.

[HoosierDaddy]

I am using Office 365 Personal which I paid an annual fee with a credit card almost a year ago. I have an unused one year license key I want to use to add another year. I searched on Microsoft site and was prompted to chat with an agent who said I should let him extend my license for a year but I was just researching then and didn't have the license key handy. I would really rather do this myself anyway. After I dug out the key, I went to my account, and found where I could add the key but there was no indication it would extend my license rather than just start the year clock over.

So basically, I would like to know if anyone has actually extended their license by adding a new product key to their account without loosing the remaining time already paid for. In theory, I should be able to add the key and turn off automatic renewal which is currently set to my CC.