whole new ballgame, a - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A completely altered situation, as in It will take a year to reassign the staff, and by then some will have quit and we'll have a whole new ballgame. This expression comes from baseball, where it signifies a complete turn of events, as when the team that was ahead falls behind. [Colloquial; 1960s]