Switching SIMs

That might work, but there's no way to be sure unless you were to call ATT and ask or just try it. Worst case scenario is that you try it and something doesn't quite work right and you have to make a trip to ATT anyway.

What I would do is upgrade her line and when the phone comes in call att and have them activate it on your line, which will then deactivate your old sim in your 4. Then just take your old 4 in to ATT and have them giver a new microsim to put in the 4.

 

Should work without any issues. I had an iPhone 4 and upgraded my wife's line from a 3G to an iPhone 4s on launch day. Activated it and she used it for a week before we swapped phones. All I had to do was swap the SIM's and restore the phones with each other's back ups. Everything worked great w/o any issues.

No need to call or deal with AT&T unless you want to get the 4S data for your line after to switch to the iPhone 4s. The 4s data plan gives the iPhone 4s faster data speeds than using the iPhone 4 data on the 4s. If you want to get the data plans switched, you need to call AT&T and have them swap the IMEI numbers on the both lines and you'll be good to go.

 

When the phone arrives just take out her sim in the new 4S and put yours in and use it. My brother who isn't even on our family plan used my moms upgrade to get his iPhone 4S just took out the sim card and replaced it with his and used it just fine.