According to an Apple representative that spoke to Engadget, existing A1428 phones cannot be updated to add AWS support.
On April 12, which is the date that T-Mobile's iPhone 5 launches, Apple will be selling the new version of the A1428 iPhone. This phone will be available unlocked at full retail price through Apple, and will function with both the T-Mobile and AT&T networks, as well as on many LTE bands around the world.Apple's enabling support of the AWS bands from the factory on the new guy - and sadly, Apple affirmed to us that it's not something that can be enabled via a simple software update for A1428 iPhone units already in circulation. To say that another way, existing iPhone 5 owners on AT&T cannot simply apply a software patch to have AWS support added. You'll need to buy a new phone next month.
Apple plans to phase out the existing A1428 hardware in favor of the new version, which means future phones from AT&T might also ship with AWS support included. While the current A1428 phone works with T-Mobile's network, it is notably slower than the modified version.
Apple's iPhone 5 with AWS included will be available at 12:01 a.m. on April 12 from 24-hour locations, and phones ordered from Apple's Online Store can be unlocked (after full payment) through T-Mobile.
Article Link: T-Mobile's iPhone 5 is a Tweaked Model A1428 Phone with AWS Support