Nokia received permission from the FCC: ID QMNRM-860 (Approved Feb 22, 2013) for an in the field software upgrade that would enable LTE band 4 (UL 1710-1755 Mhz, DL 2110-2155 MHz) on the Lumia 928. I contacted Nokia Care and received the following reply.
"I hope for your kind understanding as for now, there is no such information available in our end regarding the radio software upgrade enabling AWS on the Lumia 928." Josh (10:07:56)
I have posted on the Nokia Discussions forum concerning this and will post any responses I get here.