The new T-Mobile Samsung Galaxy S III LTE isn’t meant for existing S3 customers. It support for HSPA+ over the 1900, 1700 and 850MHz bands. It’s meant for those who want to save money on a high end Samsung phone and want LTE as well but aren’t willing to buy the more expensive Galaxy S4.
I’m speaking about the fact that the LG Nexus 4 already had an LTE radio, but was turned off by Google in its latest update because it wasn’t approved by the FCC. All it had to do was get the phone re-approved.