NCO to officer is not all that unusual. My senior DI at PI, a staff sergeant E-6, retired as a Marine captain O-3 after 20 years.
Oh! When I was in a looooong time ago (army), it was very rare. Which is why I should be more careful when talking about the situation today, or talking about the different branches of the service!
A quick search shows there were at least some during Vietnam.
Marines:
“”From 1963-1973 the Marine Corps Commandant appointed a permanent
Board with the mission of selecting those enlisted men of the Marine
Corps whose performance under fire while serving in Vietnam merited a
commission. A list of 62 enlisted men who were commissioned includes
one man who was killed before he could accept his commission.””
Army:
“”The Department of Defense, in an official communication, has stated
that there were no battlefield commissions awarded by the Army during
the Vietnam conflict. In September, 1985, a letter was written to
General William Westmoreland who commanded all troops in Vietnam
requesting the General’s comments on the subject. General Westmoreland
replied: “We did commission several NCO’s on the battlefield in
Vietnam.””