Is it possible to activate a new sim card on an existing GoPhone accont without having to buy a new rate plan?
Yes - just visit your local at&t store (or contact customer service) and let them know you have a new SIM card, and that you need the existing account information, balance, and phone number transferred to the new SIM.
(Since you lost your SIM - if you haven't already obtained a new one, they can provide you with a new one in the at&t store. You may need to add funds to your account to avoid being charged for the replacement SIM if you take this route.)
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.