What I understand is that the KC135 fleet was purchased in a relative short period 1957-1965. Ages of all the KC135 are all in the 40-47 yrs bracket.
410 of the KC135 are reengined and offers superior performace/maintainability compared to the ones flying. It can offload 50 percent more fuel, is 25 percent more fuel efficient, costs 25 percent less to operate and is 96 percent quieter than the KC-135A.
157 Air Force Reserve and Air National Guard tankers were re-engined with the TF-33-PW-102 engines. The re-engined tanker, designated the KC-135E, is 14 percent more fuel efficient than the KC-135A and can offload 20 percent more fuel.
In current plans the last KC135 will leave the fleet at around 2040. These are probably the CFM56 modified aircraft.
Why not modify part of the non reengined / TF-33 aircraft just like the modified KC135R/T with new engines and improved communications, navigation and surveillance equipment (to meet future civil air traffic control needs) ?