Trading AT&T upgrades for a cheaper data plan?

I'm not very knowledgeable about the subtleties of cell phone plans and providers. I am on an AT&T Family Plan with two other members of my family. My brother and I are both eligible for upgrades, and have selected the phones we want. He wants a Samsung Impression, and I want a Nokia E71.

When I researched the E71, I found out that if I were to buy the unlocked version I could add a data plan to my line for around $15/month, as opposed to the $30/month that a smartphone data plan usually costs. I assume that this is because AT&T does not classify the unlocked phone as a smartphone, for whatever reason, and so I would pay the regular phone data plan price. However, the unlocked E71 is out of my price range.

My plan is that my brother selects the AT&T E71x as his upgrade, and I select the Impression. Then when the phones arrive we trade, allowing me to add the regular data plan for $15/month.

Basically I want to know whether or not this will work. Will AT&T somehow know we switched phones? Will the data plan that I buy on my line (ostensibly for the Samsung Impression) work as desired on my E71x? Are there any other downsides to this plan?

Thanks so much for your help, and sorry for the wall of text!

P.S. if this is in the wrong forum, I apologize.