Apple has published an article on how to replace an original iPhone with an iPhone 3G using the same carrier. If you follow these steps to backup your original iPhone first, and then restore the backup to your iPhone 3G, your saved SMS messages, email accounts, photos, notes, and other personal settings will be present on your iPhone 3G. Basically the step are purchase a new iPhone 3G, make sure that you have the latest iTunes software, connect the original iPhone and backup, connect the new iPhone 3G and restore from backup, after restoring the new iPhone 3G will restart and then verify that your saved SMS messages, email accounts, photos, notes and other personal settings are present on your iPhone 3G. To get detailed information for the process and for trouble shooting visit Apple’s article here.