Read the anandtech tech article plus read the FCC request by Apple. I posted it a few post above.
The 1700 WCDMA band has always been in the "old iPhone 5 A1428". It was just disabled by Apple at the carrier's request.
Once again. There is no hardware changes. Even Apple states to the FCC there is no hardware changes.
It had support for LTE on the 1700mhz AWS band before. This new one has a slight hardware change to support hspa+42 on the 1700mhz AWS band. That's the difference.
http://www.tmonews.com/2013/03/t-mobiles-iphone-5-passes-through-the-fcc-halls-on-its-way-to-april-12th-launch/
Think about it. Why would they even need to pass it through the FCC again of it was the exact same hardware as the current A1428? Also why would they increase their own manufacturing costs by replacing one model with the exact same model if they could just firmware update them? Just doesn't make sense.
That's what I think but We'll know for sure 100% on April 12th what the changes are.