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sirxipha wrote:

Well, the problem that im seeing is that doesnt the new iPhone have a different type of SIM card?, I couldnt simply switch the SIMs from my old 3Gs to my new iPhone 4 if this is the case. 

No, what you should do is upgrade the iPhone on the secondary line.  Once you have the phone, take it to your local ATT retail store and have them switch the EMEI #'s and sim info. 

You should be fine doing this, I did this myself last year.