Upgrade from non-smartphone to iPhone before contract is up?

I have a non-smartphone on ATT currently and am looking to upgrade to an iPhone (still with ATT). I have a friend's old iPhone, so I don't need to purchase the device. My contract is at the end of this year however, and I'm not sure how this upgrade would affect my data plan.

I've heard that upgrading from a non-smartphone to a smartphone while the contract is still in effect makes the data plan extra expensive, because its charging me as if I were using a non-smartphone. Is this true? Do I have to wait until the ATT contract is up just to avoid exorbitant data fees?

All fees are high for a smartphone. That's how they make their cash. Don't mess up your current line. Try this. Go in and ask to ADD a smartphone line to your existing plan and price that. Let the non-smart line just run out on it's own and then shut it off. The only kink is you will have to ask them to port your number to the smartphone and issue another number to the dumbphone. Hope it helps. Best luck.