It doesnt sound like you were accessing the right part of the customer portal.
I suspect you were on the "ordering" section. This lets you order new services, but not alter existing services.
To place a regrade you need to go to the "My services" section of the portal, and then find your broadband service. You should see a yellow "regrade" button next to it, which starts the regrade wizard.
Kindest regards,
James Sweet
http://www.zen.co.uk