A:AnswerHello, usually iPhones are very compatible, however you need the IMEI number by the side of the box to make sure this phone will work with your service provider. Unfortunately you can only get this number when you have the phone in your hands.
A:AnswerAccording to apple, you could go all the way up to 13.xxx. I am using an iPhone 7 to send you this and it is using iOS 13.2.1. The 5s on back is being left out from iOS 13 forward.
A:AnswerNo, you should be good to go once you switch the sim card and transfer your settings/pictures via the icloud - at least that was my personal recent experience.