IPhone 4s and iPhone 5 sim card?

One phone in our plan has an upgrade. I want to use it to get the iPhone 5. I have the 4s. Is it possible to buy the iPhone 5 on the upgrade number, and then switch our sim cards so i get 5 and the other line gets my 4s? Or is the sim card of the 5 a different size. Please let me know what i can do if there are any tricks i could learn. I want to keep my same number

Both sims are different. After you get your phone if you order online you will have to go to a store to get a new sim. If you buy it in store tell the clerk your are using a different number to upgrade.