How do you transfer SIM card info from an ATT phone to a new ATT phone?

Even though these networks are not compatible, there is a way to transfer the contacts, check out http://sim-reader.com.

The program is called SIM Manager, it is compatible with all types of SIM cards - 2G, 3G, R-UIM and Nextel. Once you read the data from one card, you can write it to any other supported card type.

Note that a Nextel phonebook entry has many properties (name, number, mobile number, email, etc), while GSM cards only have a name and a number - so naturally, some of your Nextel entries will be mapped to more than one GSM phonebook entries.