Hi, In order to get an iPhone 3GS at upgrade-eligible pricing, I upgraded a line on my account which is not currently an iPhone line. Currently I have an iPhone 3G. When the new 3GS that I ordered comes, can I just pop in my current SIM card into the 3GS? Then, can I use my iPhone 3G to the line that I ordered the 3GS for? Will this all work seamlessly? Thanks!