Say the red player has two metropolises (having upgraded both politics and trade to level 4). Now the blue player upgrades their politics to level 5 and steals a metropolis from red.
Who decides which of red's two metropolises will be given to blue? The significance being that the city losing its metropolis is now susceptible to sacking by the barbarians.
I've avoided such ambiguity by throwing out the metropolis tokens and painting the metropolises yellow, green, and blue. But that's just a house rule.