I live in the U.S. and upgraded from the original iPhone to an iPhone 3G. If you simply put your old SIM into the 3G, however, you will only be able to use EDGE, and the phone will not use the 3G network. When purchasing a 3G model, multiple Apple sales representatives advised me that I would need to either keep each SIM in the phone with which it shipped and call AT&T to swap the phone numbers, or order a new SIM card for my existing number.
Problem: AT&T won’t switch the phone numbers since each phone is still obligated to its own, separate 2-year contract, and ordering a new SIM will cost you $25.
Solution: I physically went to an AT&T store and explained the issue. The sales rep just had to perform some voodoo in his computer system to enable 3G for the original iPhone SIM so it could be used in the new model and access 3G services. This allows AT&T to avoid having to modify any contracts while allowing you to upgrade to an iPhone 3G and use 3G services.
Done!