Will ATT automatically upgrade my contract when you buy a new phone online?

It depends. When ordering they will normally give you an option "do you want your device activated before shipping" if you choose yes then it will be activated (your current phone will not receive service after your new phone is shipped) and your contract will automatically be updated. If you chose no then you will get just a phone in a box and you will have to activate it online or by phone, once you did your contract will be updated. The easy answer. Once your phone is activated your contact will automatically renew