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.
The AMP was only about $4 million per plane, and many of the ANG pilots are used to flying other planes with glass cockpits. Also the digital avionics are much easier to maintain, and they're more reliable. With so many parts getting replaced on the 52 C-5's going through the RERP, there will be plenty of parts that can be refurbished as spares for the remaining C-5A's.