I just got the samsung galaxy but recently found out us cellular will be getting the iphone later this year?

Okay so yesterday I got the samsung galaxy s4 BUT I've always wanted the iphone which uscellular is coming out with soon. So the first point is how long would my contract be until i can get a new phone be and the second point is if my contract for a new phone is not up by the time uscellular comes out with the iphone is there a possible way to trade phones or will i have to wait until im elidgable to get a new phone ?