[Paleo Conservative]

All of the active, and most if not all of the reserve and ANG aircraft will be brought up to the M standard starting with the B and C models

No, the USAF cancelled the RERP for the C-5A fleet. Only the B and C-models will be reengined. The one C-5A that underwent the RERP for test purposes will be the only one to be reengined. All the C-5A's remaining in service will undergo the AMP to replace the existing analog avionics with a digital avionics and a glass cockpit. I'm sure that with 52 planes undergoing the RERP and 14 of the eariest C-5A's being retired, there will be lots of spare parts available to keep the C-5A fleet going for at least a couple of decades.

[2CAVTrooper]

Ah, I found the article on Airforce Link.

They axed the planned upgrade of the A model due to projected cost overruns and an uncertainty that there will be any benefit to investing in the A model.

IIRC, the A's still go through the first phase which upgrades their avionics to modern standards.