swapping sim card

in the next few months i am planning on purchasing am iphone from a 3rd party source ( gazelle, ebay, etc). i know when upgrading with one straight from att they send you a new sim bt i dont plan to do that. i know that that newer model iphone uses a different sim. i would like to know if i can purchase an iphone from a 3rd party, get a new sim card that will fit the new iphone, and activate the new iphone but stay on my same line and information. if i can order a replacement sim for a new phone, what should i expect it to cost( for an iphone 6). i dont want to purchase this new phone and not be able to get a new sim card to fit it and activate it on my line and info. thanks!