"modify so as to incorporate changes made in later versions of the same model," 1954 (U.S. Air Force), from retro- + fit (v.). Related: Retrofitted; retrofitting. As a noun, "modification made to a product," 1956, from the verb.
They car companies retrofitted all the old models with new carburetors
The mansion was retrofitted with modern plumbing
The laboratory retrofitted to meet the safety codes
the court ordered a retrofit on all automobiles