You can! Unfortunately, at present, you cannot upgrade or downgrade yourself. Instead, we need to perform those actions for you. Please drop our support a line via email (firstname.lastname@example.org) for your options.
You can see our packages here: http://www.name.com/vps-hosting ... let us know which you'd like to switch to and we'd be happy to help! Note that we are able to prorate the amount it'd cost to go either up or down and our support folks should be able to advise there.