Yes - call customer service (1-800-331-0500 or 611 from your cell phone). At the first IVR menu prompt, say "more options" and then at each subsequent prompt, say "customer service". You'll eventually be connected with a live representative who can simply change the prepaid account type for you.
However, be aware that at&t is no longer offering the PYP plans, so once you change to PayGo service, you won't be able to go back. Most importantly, make sure the PayGo coverage will be adequate for your needs before you make the change, as the coverage is not the same in many areas between PayGo and PYP service. (You may want to purchase a new PayGo line of service (SIM) with a $15 refill to use for one month as a "trial" before making the change in order to ensure the PayGo coverage will be adequate since there is no going back to PYP.)
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.