Transformers 2 Revenge of The Fallen Question

I know this sounds so simple but I cannot find were you change the difficulty on ROTF.
I tried and all I did was write over my good normal save with another normal save.
I know there are diferent settings because it says normal and there is trophies for completing it on expert.
So please can someone tell me how to change the difficulty settings.
Do you have to complete something, or get all the medals or all missions with medals.
Someone please tell me.
I tried and all I did was write over my good normal save with another normal save.
I know there are diferent settings because it says normal and there is trophies for completing it on expert.
So please can someone tell me how to change the difficulty settings.
Do you have to complete something, or get all the medals or all missions with medals.
Someone please tell me.
