My brother broke the screen to his iPhone 3GS so I gave him my old iPhone 4. I've done this manual upgrade several times myself, but I wanted to write it out and see what the MacRumors Forums community thought, just to double-check my instructions and make sure they're readable to a regular non-techie. 1. Get both iPhone 3GS and iPhone 4 in hand. 2. Restore iPhone 4 to factory settings if necessary. 3. Plug iPhone 3GS into computer and back up via iTunes. 4. Go to Settings -> General -> About and note the IMEI if the SIM card in the iPhone 3GS. 5. Note the IMEI number of the new, unused micro-SIM that will be used going forward with the iPhone 4. 6. Call your service provider and tell them the cell phone number you wish to upgrade, confirm the IMEI of SIM card in your 3GS and read off the IMEI of the new micro-SIM card that you want them to transfer service to. They will deactivate the iPhone 3GS SIM card and activate the iPhone 4 micro-SIM. 7. Restart your iPhone 4 and confirm it gets service. Make a call, send a text, and use mobile data to make sure everything is in order. 8. If desired, restore the iPhone 3GS to factory settings. Keep the old, deactivated SIM card for activation purposes when you reset or restore the 3GS via iTunes. Thanks for any corrections or suggestions!