How much does it cost to switch plans from two iPhones?

OK, so I suck with this stuff, so I'm not sure how to do this.
I just lost my 3G (got stolen) so now I'm looking at Walmart 4S, selling at $40 (that seems almost to good to be true, so also if possible could you validate that for me?)
So what my dad and I are wondering is how much does it cost to switch from one phone to another? Assuming I buy the 4S and switch the same plan and number from it. I have an AT&T plan, so if I could get a price estimate for how much it would cost to switch, or if it would even cost anything to keep the same plan but switch it to a different phone, that would be great.

Call at&t and get the answer, best method

If you have an upgrade its 40 bucks but if not it could be around 500 bucks

Actually you won't be able to get that price $40 for the iphone 4s. You are currently on a contract and have service with A T&T. That price for the iphone 4s for $40 at Walmart is for new customers that sign a 2 year contract or their contract has already ended.

Since your contract has not ended with A T&T, you would have to pay full retail price without any discounts for another phone. Full retail price without any discounts would cost you about $600.00 for a brand new iphone.

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