Can I turn an iPhone into a prepaid phone?

So on ebay I'm considering buying an iPhone. Its an iPhone 5 with ATT. I've heard you can put a sim card in and change it to prepaid, but I have questions about that.

1. Does the phone have to be unlocked to change to prepaid?

2. If I change it to prepaid will it work with any other prepaid company (Verizon straight talk etc.) or will I only be able to use ATT prepaid?

3. Is it hard to change sim cards? Would I have to take it in to a phone store and pay for it to be done or can I do it myself? Also if it does have to be unlocked will I have to take it into a phone store and have it done?

Call AT&T as policy's change daily

1. Yep.
2. Since 4s all iPhones come dual band. Meaning they can use GSM like ATT or CDMA signals, which is more like a lot of prepay carriers.
3. Unlocking will probably cost you a buck or two. Only the original owner could call ATT and get it done otherwise. Prices vary and it can take weeks because it has to run back through Apple's servers. Your new carrier should be able to give you a SIM free or for a few bucks max.
See below. Best luck.