If you click on "My Service Plan" under the "My Services" header at the top of the page (you may need to sign in again), then click on Change Plan where it shows your current plan, you should be able to change your current plan to the one you wish, as long as it shows as available.
If this doesn't work for you, I'm sure a rep will reply. They're on M-F from approximately 8AM to 5PM EST. It can take them a day or two to reply.
Edit: I should add that, as far as I know, if there is a choice of a monthly discount with a new 24 month commitment, the discount is only for the first 12 months of that new 24 month commitment.