question on upgrading on 1 line and then swapping to another

If you are on att then just order it on your line and when you get the phone take it to the ATT store and have them switch it to the other line.

 

Yup. I have 4 lines - me and the three kids - and we do the SIM swap every time a new iPhone comes out. I pay for the phones, so I get the new one and they all swap out so that no one is ever more than 2 gens behind.

We'll be replacing a 3GS this time around. Everyone will be on an iPhone 4 or better.

 

This is on the AT&T site, so you might not actually have to go to a store (obviously, the info is from last year):
My phone number is not eligible for an upgrade, but another line on my account is. Can I use it to upgrade my phone to iPhone 4S?
Yes, you can use the upgrade eligibility of another number on your account to upgrade your own device. To take advantage of this, upgrade the line that is eligible. When your device arrives, follow these simple steps and an AT&T representative will help you activate.
Contact AT&T at 1-800-331-0500 or 611 from a different wireless device. At the prompt, enter the wireless number that received the upgrade.
At the next prompt, choose option #2 to activate your phone on a different mobile number, and an AT&T representative will help you from there.
http://www.att.com/esupport/article...grade+my+phone+to+iPhone+4S?#fbid=bwYepaDET5m