The Navy, moving to tighten security in the wake of recent spy scandals, has replaced roughly half the officers and senior enlisted men who maintain classified communications manuals. The sweeping series of reassignments began in August and was just recently completed, the service said. The replacement of hundreds of so-called "communication security material system custodians" was a "precautionary measure" that should not reflect negatively on those who were reassigned, it said. There are roughly 1,250 security custodians in the Navy. The service declined to say exactly how many of the security officials had been reassigned before the normal end of their tour. But an officer who asked not to be named confirmed that the figure was about 50%.