i.e. the Mets move to the American league or the Yankees/Red Sox move to the National league.

If the leagues are significantly renamed or there are no more leagues, then this resolves YES

If leagues still exist but are irrelevant due to there being no difference and no play restrictions, this question still counts and will be resolved according to the definition.

if it happens or is announced by MLB as a definite plan with a clear date to change, it counts.

significant = it changes the day to day way you would refer to them.