...Would my current phones SIM card deactivate? I want to continue to use my current phone while I wait for the new one to be delivered! How does this work? Do I have to activate the SIM card that comes in the mail?
No, your current phone won't deactivate. It will continue to work just fine. When you receive your new phone just log into your account and "swap" your phones. Then you can just take your existing SIM (if it's the same size) and put it in your new phone.