West stand lower is £24.00 to upgrade, painless to do at the ticket office and they will give you a paper ticket if you want.
As it so happens, I rang the so called 'season ticket customer care' today about something.
As an afterthought, I asked the same question.
The guy said it was a straight, no nonsense £14 to upgrade, whether over the phone, or even just before the game, via the Amex in person.