The changes will take effect for best-of-seven matchups this October, a person familiar with the format told The Associated Press.
The extra umpire will work home plate in Game 1, go to right field in Game 2, then shift to the replay booth in New York for the rest of the series. Another ump will handle replay the first two games before working the remaining games on the field.
The person spoke to the AP under condition of anonymity because umpire assignments haven't been announced for the World Series and the AL and NL championship series.
MLB began having six umpires on the field starting at the 1947 World Series, when rookie Jackie Robinson and the Brooklyn Dodgers opened at Yankee Stadium.
Six-man crews will call this year's wild-card games and the best-of-five division series. The post-season begins Tuesday night when the Oakland Athletics visit the Kansas City Royals for the AL wild card.
The plan for seven-man crews came from bargaining between the umpires' union and MLB this year when replay was increased to review most every call except balls and strikes. The extra slot gives MLB another way to include a first-time ump in a high-profile environment.
Under the new arrangement, an umpire who starts a seven-game series in the replay booth would work Game 3 in left field and eventually rotate to the plate for a potential Game 7.
As part of the agreement, a replay umpire will be assigned to every post-season game, with another umpire serving as his assistant. The assistants will remain in the replay booth for the entire series.
Before 1947, four-man crews worked the World Series. The AL and NL each supplied one alternate umpire to be at the games in case of an injury or illness.
From 1947 through 1963, there were six-man crews at the World Series. But the umpires who started on the left field and right field lines stayed in the outfield for the whole series, while the four umps in the infield rotated spots each game.