In September 1967 the Diplomat and its less expensive siblings received a light update consisting of rub strips on its flanks, new recirculating ball steering by ZF, and a heated rear window.

In March 1969, Opel introduced a new line of KAD models with new bodies and a more sophisticated chassis with a De Dion tube rear axle.

The new body was a typical General Motors style and bore a strong resemblance to the contemporary Holden Brougham from GM Australia.

In October 1971 the Diplomat's list of standard features included H1 halogen headlamps, radio antenna integrated into the windscreen and remote-controlled rear view mirror.

From January 1972 the 4-speed manual gear box was no longer offered on the Diplomat E. September 1972 saw the introduction of a modified grille with OPEL script relocated from its left side to its central portion.

Opel continued the production until 1977 but found little success in the market: only 61,559 KAD cars were built from March 1969 to late 1977 (of which 4,976 Kapitäns, about 33,000 Admirals and about 23,500 Diplomats).

But the precise panel gap tolerance found in Diplomat was beyond what the GM North American manufacturing plants could do without extensive and expensive changes to the sheetmetal stamping and welding process.