Buying a phone with a different sized sim card and swapping the sim effect a contract?

So I have an old Galaxy S 3, Just about obsolete and it's getting to the point where the phone take 5 minutes to load things that my Wife's Tracphone loads in a couple of seconds. So while I would love one of latest big awesome phones, I don't feel like shelling out an entire paycheck to buy one outright and I am still on my parent's phone plan while we get our feet under us. So to make due i wanted to buy an at&t prepaid phone and swap my sim card into it. One issue, I have a Micro sim card, Most the phones now use a Nano sim card.

So I know to get a new sim card at the nano size I would need either one of my parents with me, need to be added as and authorized user, or need their pin code. I also know there is a charge for the new sim. That said, my parents are on a old contract and don't want to renew it which with their contract it's one that if they use the available upgrade it will renew the contract for 2 years. 

My exact question is if I get a new nano sim to use in a prepaid phone would it make any changes to, or, renew the contract that my parents have?