Can i switch iphone sim cards?

Ok well me and my mom both have iphone 4's i have a black one (8 gb) and she has a white one (16gb) i use more apps ect… She is willing to trade sim cards but will it affect my contract? If it makes any sense

No, this will not hurt anything. You are able to phone swap as much as you want. It should be as simple as just moving the card over.