Most of the mechanical systems are being upgraded with off the shelf commercial technology. The APU's are also being replaced. All the aircraft have to first go through the AMP to upgrade the cockpit and avionics before getting the RERP. The cockpit upgrades especially will help improve maintainability due to commonality of the computers with the 777 and other commercial aircraft. Instead of having lots of obsolete mechanical gages manufactured by companies that no longer exist requiring the skills of a Swiss watch maker to repair, it will have several identical LCD screens that can easily replaced if one fails. Even if one screen fails in flight, the instrument displays can be moved to another screen. The whole idea is to use commercially available parts to cut down on costs and take advantage of technical advances available to the airlines.
Lockheed is guarantying a 75% mission capability rate, but some think the reliability might get as high as 85%. Effectively even with the retirement of 14 C-5A's it would be like having and additional 20 C-5's available. The increase in thrust will allow an increase in the maximum takeoff weight and effectively increasing the amount of lift available without increasing the number of planes.
http://www.globalsecurity.org/military/systems/aircraft/c-5-serv.htm