Not trying to be redundant but this is cut from my other post about "why AM"
As far a changing to digital comm as others have mentioned. The Nexcom program is replacing FAA VHF radio equipment nationwide. These are dual-mode equipment, capable of both conventional AM and digital modulation (D8PSK). The digital mode will support both digital voice and data services. From what I have gathered, the digital modes will be first introduced in the super-high sectors because the aircraft that fly in those sectors have more modern systems are therefore more likely to be able to use the digital modes. The MDR (multimode digital radio) equipment is capable of 25 KHz and 8.33 KHz channel spacing.
There is a simultaneous but unrelated project underway to replace the UHF comm radio equipment. These use conventional AM and are not digital capable and they are only capable of 25 KHz channel spacing.