The addressbook is usually saved in the device memory and not on the SIM card. If you create the backup from the 8700 you can replay it on the 8310. Just make sure you are using Desktop Manager 4.2.2. If you restore the backup to the 8310 you will have the same data (addressbook, messages etc.) on the new device.
If you are using BIS, you have to insert your old SIM card in to your new device and make sure the device is connected to the network. Then login to the BIS website and switch the device. There is no need to call ATT to do the activation.