Apple has published a Knowledge Base article that contains a step-by-step process for replacing a first-generation iPhone with an iPhone 3G while retaining the same carrier. The process allows uers to preserve their SMS messages, email accounts, photos, notes, and other personal settings.
The article notes that in the US, SIM swapping is not necessary. Data from the original iPhone will be transferred to the new device's SIM. In other countries, you can use your original iPhone's SIM with iPhone 3G.