Do I need to jailbreak my iphone 4s to use it with another carrier?

I recently bought a Verizon iphone 4s from someone and I want to use it with AT&T. I know I need a smaller sim card but do I have to jailbreak it and unlock it? If so, can you tell me somewhere that isn't scam that could do it for a low price or if I can do it myself?

Sprint and Verizon iPhone 4S phones support both CDMA and GSM networks. But preferred is CDMA. In order to use them on GSM network you have to jailbreak your CDMA iPhone 4s. Once jailbreak is done, go ahead and perform following couple steps to disable CDMA modem in iPhone 4s:

- Connect your 4S with iFunbox or any other iPhone file manager to PC
- Go to ar/wireless/Library then delete file carrier_310VZW_20404_11.2.plist
- Go to /System/Library/Carrier Bundle/iPhone delete folder Zeppelin_US.bundle
- Delete your sim card carrier bundle in the same folder ( if your iphone is Sprint so delete folder sprint_bundle - NOTE. In some phone this folder does not exist, just skip this step)

It's done, now your CDMA modem does not have PRL (preferred roaming list file) and will not be able to search and register on CDMA network.

Program your gevey with correct settings and enjoy your unlocked iPhone 4S.
I have personally tested 2 Sprint 4S in US and got them unlocked with V13 update.

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P.S. This instruction might not be required if you unlock your iPhone 4S outside of US.
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Call Verizon and pay some money to get offical unlocked
http://iphone.pandaapp.com/tutorial/