Log out and back into your account and you should see the change.
I have a few times, but no change I wonder if it could be because it would not be a 30 day extension but a full upgrade as I have not been here or paid in quite a while?
I know its an old thread, but this same thing happened to me, was a premium a few years ago...
Code worked and said expiration was in xx/xx/0001, however it still says basic on everything
edit: My group says premium on the forum, does that revert once your premium account lapses? or are the sites just out of sync
edit2: Just read on that page: "Only Groundspeak Basic Members who have not previously been a Premium Member are eligible to redeem a Free Premium Membership Trial." When I try to use another account, it says the code has already been used.