I don't think telling them on the day will make a difference as at that point, it's out of their control and you'd have to wait again to sit the higher paper. I think you could appeal but if you already got a 5 then it won't make a difference. I think sitting the higher papers externally is an option, so you will end up actually achieving the higher grade that you want. Idk, I'm thinking about students who are home schooled. I know they can sit it at another school or at a test centre.