I used to buy almost exclusively from Wal-mart, as the price was typically $50-100 cheaper and they worked, at the time, under the same contractual rules as COR. I had done several types of transactions there that are outside the norm.
A few months ago we went to use the upgrade on one of of our lines. We explained everything to one employee who got the phone out and instructed another to finish our transaction. After they had already unboxed the phone completely and half way through the computer transaction they informed us that Wal-mart no longer allows for Cross-Upgrades. That is upgrading one line and actually using the device on another. Due to the needs of individuals on our plan we NEVER use the upgrade for a line to buy a phone for that line specifically. So we went to leave and a manager happened to see us leaving the fully unboxed phone and came to ask about it. He was apolegetic and could do nothing to remedy the situation.
It was explained that Walmart had altered their mobile phone policies about the same time they started building the dedicated "store-inside-a-store" mobile centers instead of having a kiosk in electronics (Atleast in the South here).
My opinions are my own and do not reflect the opinions of my employer AT&T.