Complex sentence: By May 1968, the wing had upgraded to the F-4D aircraft, and then in 1969, two additional squadrons of F-4Es (4th, 421st TFS) joined the wing.
Split into simpler sentences: By May 1968, the wing had upgraded to the F-4D aircraft. In 1969, two additional squadrons of F-4Es (4th, 421st TFS) joined the wing.