If it were easy, everyone would do it. Play baseball, I mean. No, I mean quit a starting pitcher role and turn into a veritable ace in relief.
Starting pitching has always been where it’s at, and this is still true (albeit to a lesser extent) today. The great Baltimore teams of the late ‘60s had the best bullpen in the game, with guys like Stu Miller, Dick Hall, Eddie Fisher, and Moe Drabowsky. But well into the ‘70s, complete games remained common enough (for instance, in 1977 Jim Palmer and Nolan Ryan tied for the AL lead with 22) that bullpen work was considered a demotion.