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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.