Switch from iPhone 4 to Note 2

I currently have a iPhone 4 under contract with AT&T, and I was wondering how/if I could switch the sim card to a Note 2 and continue the same contract from AT&T on that new phone? If so I would like to purchase a Note 2 (been interested in one for a while) and put my sim card in there to use as my main phone. Also, whenever my family upgrades their phones, could I just put the new sim card into the Note 2? Thank you in advance.