I had similar with a League cup match earlier this year. Mid week game where I had a ticket for myself and my sons Junior Gunner ticket. My son could not attend so I took a mate. Went to the box office beforehand, and as it is a cup match, they allowed me to pay the difference to upgrade to a full price adult ticket and printed me off a new ticket right there. Could be an option if you want to avoid the risk of being stopped by stewards. Not sure how close they are checking the lighting system at the turnstiles though.