I went into an AT&T store to get my mobile share plan updated from an iPhone 4s plan to a 4G plan so that I could get HSPA+ data speeds. In doing so, I gave him my IMEI number and he entered in and it selected the correct data plan. He didn't say anything other than 'You're all set", so whatever he did worked. I rebooted and did a speedtest and my speeds went from 4.30Mbps to 10.14Mbps. For kicks, I downloaded speednet on an iPhone 5 in the store and ran it and it go 10.30Mbps. Thought that was interesting that the HSPA+ and LTE speeds were identical.
That said, I don't know if the AT&T rep I dealt with just knows what he's doing and I got lucky or if the phone was added to the system.