a whole new ball game

a (whole) new ballgame

A situation that is completely different from what one is used to or expecting. Living away from home is a whole new ballgame for most young adults. I used to babysit, but having a child of my own is a new ballgame.
See also: ballgame, new
Farlex Dictionary of Idioms.

a whole new ball game

or

a different ball game

If you describe a situation as a whole new ball game, or a different ball game, you mean that it is completely different from what came before. I'm working with kids now, which is a whole new ball game. If military force were to be used, then that would be a completely different ball game. Note: `Ball game' is often used in American English to refer to a game of baseball.
See also: ball, game, new, whole
Collins COBUILD Idioms Dictionary, 3rd ed.

a whole new ball game

a completely new set of circumstances. informal
The phrase originated in North America, where a ball game is a baseball match.
1989 Looks Making the film was a whole new ball game…for Kylie.
See also: ball, game, new, whole
Farlex Partner Idioms Dictionary

a whole new ball game

n. a completely different situation; something completely different. Now that you’re here, it’s a whole new ball game.
See also: ball, game, new, whole
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • a (whole) new ballgame
  • a different/new ball game
  • ballgame
  • new ball game
  • whole new ballgame, a
  • a totally different ballgame
  • house and home
  • right you are
  • right you are!
  • adulting
References in periodicals archive
Burk, Never Just a Game: Players, Owners, and American Baseball to 1920 (Chapel Hill, 1994), Goldstein, Playing for Keeps, 134-149; Allen Guttmann, A Whole New Ball Game: An Interpretation of American Sports (Chapel Hill, 1988), 61-69; William A.
(27.) See Burk, Never Just a Game, 207-209; Guttmann, A Whole New Ball Game, 65- 69; Lowenfish, Imperfect Diamond, 88-90; Seymour, The Golden Age, 212, 230-34; and Voigt, American Baseball, vol.
At the Nexus of labor and leisure: baseball, nativism, and the 1919 Black Sox scandal
Even though he knew a lot about collecting, he found himself in a whole new ball game when he started Ultimate Line-Up Inc., the only African American-owned company licensed to produce Major League Baseball and National Hockey League card games.
Trump card
Baker, Sports in the Western World (Urbana, 1988); Allen Guttmann, A Whole New Ball Game: An Interpretation of American Sports (Chapel Hill, 1988); Melvin Adelman, A Sporting Time: New York City and the Rise of Modern Athletics, 1820-70 (Urbana, 1986); Richard Mandell, Sport: A Cultural History (New York, 1984); and Benjamin Rader, American Sports: From the Age of Folk Games to the Age of Spectators (Englewood Cliffs, 1983).
Negotiating the "folk highway" of the nation: sport, public culture and American identity, 1870-1940