Your current phone will not work. Ever. Boost is CDMA. AT&T is GSM.
My buddy was on AT&T's prepaid plan and he paid $50 a month for everything included. But he's been off it for over 6 months and is not going back. Do your research. He didn't have an automatic setup, he paid by buying pins at a store.
You can buy any gophone at any store and activate the phone. Once activated you can take the sim and put it into any AT&T or unlocked gsm phone and it'll become your phone. Meaning, buy a $10 (or cheaper) go phone, activate it, take the sim out and put it into a fancy android phone. The android phone is now your phone. GSM works differently. They send service to the SIM. If the sim is in a dumb phone, its your phone, if the sim is in a smart phone, its yours. The notable exception, as I understand it, is the high end phones and 4g phones. They marry the phone to the sim they sell it with to keep people from swapping. The only people who can unmarry them is AT&T.
Good luck.