What would happen if i changed my sim card from my iphone 4s into an iphone 4?

I just switched an active sim card from my active iphone 4s into an unactive iphone 4. Now my iphone 4 is active and programed with my plan. I just want to know if AT&T (my company) is going to charge me more because i switched the sim cards. And note that i did this all by myself.

No, Att really dosent care which phone you use as you already have a data plan with them. Typically i use my sim card to test many phones through out the day as i'm the owner of a couple phone repair shops in Florida. Hope that helps

No, they will not know. Why did you change from the iPhone 4S to the 4? It should not make any difference, but ATT is probably taking advantage of you, because the iPhone 4 plan is probably cheaper, so you are paying for the 4S, but you get the 4.