bocadillo
Water Gypsy
I spoke to the ticket office last week and they said that if you have missed a Platinum fixture (so far, 'Boro and Derby), you can exchange for another Platinum fixture.
If you wish to exchange a Gold fixture, you can, on payment of the difference in the ticket price between a Gold and Platinum fixture.
Seems fair, but it's a pity that they don't advertise it on the website.