Will ATT store transfer number and plan from iPhone 4S to eBay 5S?

I have been with ATT for well over 10 years and currently have two iPhone 4Ss on a 3GB shared data, unlimited voice and text plan. The phones are getting tired and I would like to replace them with used iPhone 5Ss (ATT) from eBay. I understand the SIMs are different.

Can I take the old and new phones to a local ATT store and have our existing numbers and plans moved from the iPhones 4Ss to the used iPhones 5Ss? Do either the 4Ss or 5Ss need to be opened for the swap? Or is it a simple matter of programming? Will ATT charge a fee for the swap? We expect to stay with our plan for the foreseeable future, so a 2-year committment is no problem, if required.

Thanks,

-stephen