Question regarding Best Buy Mobile "early eligible" upgrades-?

Today I got a phone call/automated voicemail message from Best Buy stating that my mobile device (an Apple iPhone 16GB 3G-S) was eligible for an early upgrade. This is not the same thing as the full upgrade. It says that I can upgrade it to the latest iPhone, but nothing else. My question is: What exactly is this saying? Can I really go and get an iPhone 4 upgrade for the price of the phone itself? How much would this actually cost?