How to unlock a a Verizon iPhone 4 to be used on AT&T?

I'm buying a Verizon iPhone 4 from my friends mother and I'm on on an AT&T plan. Is there anyway I could unlock the Verizon phone so I could use it on my AT&T plan?
Also, please don't mention jailbreaking, for it is illegal now.

Added (1). And if so, how do I go about it?

I'm very sorry, but verizon phones canmot be used on att. No matter the circumastances.see, verizon uses cellular technology called CDMA, which means it doesn't use sim cards. I don't know if you know what sim cards are, but they are chips that are inserted into att or tmobile phones so they can activate the phone so you can call/text/browse the web.att uses gsm, which means it uses sim cards. So sorry, but you can't. There's no slot for a sim card on verixon phones. No way to jailbreak.verizon is fricking stupid, good choice for using att.take care.

You would have to get a new SIM card.
Rather than buying a new card and then installing it, it would cost about as much then just buying a new iPhone.

Short answer: You can, it's just cheaper and less time consuming to do so.

How do you use your CDMA phone on a GSM network?

I suppose you would probably do so by completely taking the phone apart and re-manufacturing it with GSM technology, including adding a SIM card slot. Since nobody has ever done this, including technical geniuses, I would be wary of trying it. Even Apple just makes it for one network or the other right from the start and doesn't mess with it from there.