OK so currently I am not due for an upgrade, however, my sister is due and said she would be willing to give me her upgrade if I give her my current iPhone 3G. Is this possible through AT&T and/or Apple to use another persons upgrade for yourself? Also, on another note, recently my 3G has been randomly coming up with the message of "No Sim Intalled" takes a few seconds and then it goes away and my service comes back. Any tips? Should I just take it back and see if Apple will replace it?