I'm not entirely sure. I think if you just mentioned a light leak they may think it an unreasonable return reason, you could always 'find' another fault to accompany this, something like your touch screen stops working?
I'm not sure if they send it to you first with iPhones. if it's anything like when i replaced my macbook charger what they did was to take my card details and send out the new charger with a prepaid packet, i then had 10 days (or two weeks, can't quite remember) to send back the old charger, and if i had not returned my old charger they would have billed me for the new one.