iLounge is reporting that some people have received incorrect information when contacting AT&T about upgrading their 4GB or 8GB iPhones to the new 16GB iPhone. In a recent thread on the Apple Discussions, an Apple employee "Nathan C." said that you can just replace the SIM with your current iPhone's SIM. He later updated his post saying, "My apologies these steps may not work. You may need to activate it with the new SIM choosing the option to "replace" an existing phone on your account."
Mark Siegel who is the Executive Director of Media and Analyst Relations with AT&T emailed iLounge to tell them that if you choose to upgrade your iPhone from 4 or 8GB to the new 16GB, your contract will be backdated to the starting point of your original iPhone's activation date. He went on to say that iPhone customers should use the SIM that came with their iPhone.