Most of those parts of the VHF & UHF bands that are used by FAA and DOD are not under FCC control. Freq ranges under government control are sliced up by several differnet controlling agencies including NTIA, NCA and IRAC. Each has a role in the decision as to how things play in those segments. Basicly as far as aviation is concerned, they have authorized these bands for FAA and/or Dod use and they leave them alone to slice it up as needed.
The only jurisdiction that FCC has in those freq ranges are for licensing any civilian organization that has a bonafide need to operate there. In those bands, the FCC regulates licensing for example the fixed base operator at an airport. But it does not regulate how the FAA spaces channel assignments. When FAA switched from 50 KHz channels to 25 KHz channels in the 70's, it was an FAA decision not FCC. Dito with the switch to 8.33 KHz. As far as DoD @ UHF, they may already be operating on 8.33 KHz spacing but as far as air traffic control is concerned, there won't be a switch to that spacing.
Dan