If you walk up in person if you go in person to the box office I'm sure they'll exchange your tickets
I agree with Thomas. They are very nice. Just exchange them.
You should read your return policy, usually if you go to the customer support desk in person they will issue a certificate which is good for a person for a specific time limit to attend any time in that period and interesting is if you purchased let's say $ 13 ticket with that certificate you are allowed to attend to any movie even 3D or etc...
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