How to Change Users on a 3GS iPhone

I would erase all content and settings on your 3GS.

After doing so and with each iPhone powered off, insert the SIM card from your wife's 3G into the 3GS. Connect the iPhone to iTunes which should provide the activation process to activate your wife's SIM card with the 3GS along with associating the 3GS IMEI number with your wife's SIM card and line/number.

When you receive your new iPhone, activate it through iTunes to put your phone number on it. Have her sync and back up her 3G on her iTunes and check all of her voicemails (you will probably want to also do this with your 3GS before activating your iPhone 4). Then, take the SIM card out of her 3G and put it in your old 3GS. The SIM tray on the 3G and 3GS is located on the top of the iPhone between the headphone jack and the sleep/wake button. You can use the SIM tray "key" that is provided with your iPhone. To open the tray insert the key or a paper clip into the small opening and push until it pops out slightly-do not turn it or try to pry it open. Pull the tray out and swap the SIM cards. Once her SIM card is in the 3GS, connect it to iTunes and activate it and sync it. You can use the iPhone 3G as an iPod Touch and back up iPhone or sell it/give it away.