The upgrade was not limited to first line USAF units, as beginning in 1985, Air National Guard (ANG) and Air Force Reserve (AFRES) units also began trading in their Vietnam Era aircraft for newer and more sophisticated weapons systems with F-16A/Bs being allocated to Guard and Reserve units as active duty Air Force units upgraded to the F-16C/Ds models. F-15A/B models of the 1970s were also provided to ANG squadrons when newer F-15C/D models reached front-line wings. As A-10s replaced A-7s, A-37s and OV-10s in the Regular Air Force, the A-7 and A-37 close air support aircraft, along with newly produced twin-seat A-7Ks, were flown by many ANG squadrons, often training with both Regular Army and Army National Guard combat units in ground support operations. The last A-7D/Ks were retired in 1993, being replaced in the ANG by A-10s and F-16C/Ds.