I don't think you'll be able to do it online.
On a related note, if T-Mobile's officially supported band 12 list (
http://www.t-mobile.com/optional-services/coverage-phones-700.html) isn't updated by 9/12 then I don't see me staying with T-Mobile. If it isn't on that list then it matters not if your phone supports it, T-Mobile won't. Just a heads up.