Tryke buddy you are very much confused. Let me just say that 3G and 3GS are two different Iphone MODELS like the Iphone 4G you cant do a software update and think its magically transformed into the the 3GS, replace "Iphone" with any nokia for example and repeat your sentence to see if it makes sense :P

Also The iPhone 3G's processor is 412 MHz. The iPhone 3GS's processor operates at 600 MHz, 3G has 2mp cam while 3GS has 3mp cam. So you see its physically NOT possible for your 3G to upgrade into a 3GS :P