You will need a nano-SIM for your iPhone 5 which your carrier provides. You might also cut your original SIM or micro-SIM to fit in your iPhone 5. I generally don’t sign contracts. I prefer buying phones with no-commitment pricing (generally Craigslist or eBay). I had to sign a contract to get the iPhone 5 since it is not immediately available as an unlocked variant.
Logged in to my AT&T upgrade portal via AT&T premier (business discount). I was overjoyed to see the unlimited data plan that I was grandfathered in automatically be upgraded to unlimited LTE data plan for the same price, $30. I am OK with losing FaceTime since unlimited data plan trumps FaceTime any day! The new data plan options are outright crazy with 1 GB pegged at $20 and 5 GB at $50 though the latter includes tethering.