That's not necessarily true. There is little difference between WiMAX and LTE on all of the hard parts: antenna configuration, transceivers, and amplifiers. Even the modulation scheme is the same: OFDM. Sprint started installing firmware-upgradable base stations a little while ago and I strongly suspect that the process for converting from WiMAX to LTE is governed by staff time limitations than outright technical debt.
Unfortunately LTE-based devices on Sprint still won't be backwards compatible with the converted WiMAX towers as they only output in 2.3 GHz, 2.5 GHz and 3.5 GHz bands. The iPhone 5 specifically maxes out at a 2.1GHz band.
Oh ya, the user terminals out there are going to have to be replaced wholesale. My only point was that Sprint is going to be able to convert to LTE a lot quicker than people expect. Building up a user base is another issue entirely.
