I haven't done this, but my understanding is that if you do this, you'll lose any discount you had on the tickets. You can upgrade, but you'll end up paying the gate price for new ticket overall. So, it could be much cheaper just to buy the number of days you want in advance...hmm, that sounds confusing, let's try this instead:
They somehow know how much you paid for the ticket originally and only apply that value towards the gate-price of the new hopper.