Have just successfully upgraded 2 AWS enabled Galaxies S4 I337M to Android 4.4.2 (Modem NC1) from 4.2.2 (Modem MG1) through simply clicking "Upgrade Firmware" button in Samsung KIES software on Mac! Both SGS4 were rooted and unlocked from Rogers. AWS was enabled on original 4.2.2 and now works perfectly on both devices(WIND Mobile connected). All programs work well and no information is lost.
Goodluck guys! It works 100%!
The thread is really convoluted, what exact process did you use?