can I turn my at&t contract phone into a prepaid phone
just began contract
Absolutely, unless if you're still under contract you cannot.
You definitely can and you don't even need to contact AT & T for most phones. You can do it yourself by taking out the battery and SIM Card then sliding it into the cage of the contract phone. Have a look at http://www.ehow.com/how_7654683_use-att-contract-phone-prepaid.html.