The earliest sports-related reference I can find by searching newspapers.com for various combinations of rebuild and reload occurs in a column by Fred Stabley Jr. of the Lansing Times on September 2, 1973. The column is titled “Rebuild? Not Bo–He Just Reloads” and the first sentence is “It’s rapidly becoming apparent that Michigan’s Bo Schembechler never rebuilds . . . he just reloads.”
The phrase spread rapidly and was regularly being applied to various powerhouse teams, mostly college but some professional, by the mid-to-late 1970’s.