In the past, you may have considered swapping out the SIM cards to accomplish the same thing.  This may have worked due to the fact that previous phones used a micro-SIM card.  The iPhone 5 uses a nano-SIM card, however, and the older SIM card that comes with previous model iPhones will not fit.

If you are not comfortable doing an upgrade swap over the phone, you can go into an AT&T retail store and someone there will assist you.  While I successfully did this on Friday morning, be warned that AT&T is in no way obligated to swap your phone number.  I purchased an upgrade for the iPhone 4S on my own line back in November 2011. Friday morning I went to an AT&T retail store with my unopened iPhone 5 that I pre-ordered with another line on my family account that was eligible for the upgrade to have them perform the upgrade swap.  The process was quick.

The line that was used to purchase the upgrade, and the contract associated with it, will reset to two years.  There is no way to get around that.  If  line that was used to purchase the upgrade is not the line that you intend to use with the new phone, you can also handle the upgrade swap over the phone with the following steps:

• Upon receiving the new phone, you can take it out of the box, but do not turn it on and do not attempt to activate it yourself.
• Call AT&T customer service at 1-800-331-0500 from a phone other than the two phones in question.
• When prompted, choose the option to activate your phone on a line other than the one used to purchase the upgrade.
• A customer service representative will then assist you to make the switch.
• Once the switch has successfully been made, turn on the new phone and follow the on-screen instructions.

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