IPhone upgrade? Mobile plan? AT&T?

I currently have an iPhone 4s. I want to upgrade but my plan monthly payment will go up by $15. My question is. If I buy an iPhone 5s from my friend instead and just switch out SIM cards… Will my plan stay exactly the same? OR?! If I buy an iPhone 6 AT COST! With No contract and simply put my SIM card into the iPhone 6 that I bough straight out will my plan stay the same? LONG STORY SHORT- HOW CAN I KEEP MY SAME PLAN WITH NO CHANGES?

Yeah you can just move your sim card but you will have to warranty if you try to call ATT if you have an issue.