Hi Erick
I found this website I think it says that you can use the old card but you wont be able to take advantage of the 3G features of the phone. The below info was copied off the website in the link
http://forums.mobiledia.com/topic59568.html
I am changing plans and upgrading my Samsung flip phone from Cingular at the end of June. I am probably going to get the V3xx or something "3G".
I have a fairly new 64k SIM card from a insurance replacement this year and it has all my numbers (etc) on it. So do I have to use the newer "3G" labeled Cingular / ATT SIM cards to get full benefit, or can I just pop in my old SIM card?
If the new phone is 3G then you will need the 3G sim in order to take advantage of 3G services.