Here's AT&T's policy for the Lumia 900
From Best Buy's website regarding the Lumia 900: Early termination fees will be prorated, starting at $325 minus $10 for each full month of service commitment completed.
So, if you bought a Lumia 900 in mid-April, and upgraded to a Lumia 920 January 1st, you would have completed eight months of service, and the early upgrade fee would be $245.
So, a new Lumia 920 would set a launch-time buyer of the Lumia 900 back about $400 ($245 + $150 + other fees), about $200 cheaper than the unlocked price. Not too bad, considering you can still keep the Lumia 900 and use it as a small WiFi tablet, give it to someone, or sell it on eBay.