I would like to nominate te Douglas A-26 as an old warhorse. In service in 1944 in WW-2 it was retsained and redesignated as the B-26 to the great confusion of many. It then was used through the Korean War. It played a part in the Bay of Pigs fiasco being used covertly by the CIA.
When Viet Nam heated up Invader regained their old A-26 designation. No B for Bombers were allowed at that stage in Viet Nam. They got a remanufacture and an engine upgrade and served there, I would say three wars was a pretty good record.
As an aside, it has a place in my heart. My first airplane ride was in a NY Air National Guard Douglas B-26 with 18 guns. I was a Civil Air Patrol Cadet and we cadged a ride at Floyd Bennet Field from the National Guard. I was 14 years old in 1948 and was in heaven.
As a further aside, don't think there were very many warplaned that had 18 x 50 caliber guns.