Cardassians have to rebuild almost entire union.
But how much of that Union are the Cardassians going to be "allowed" to retain?
Any previously disputed territories would go to non-Cardassian powers, like the worlds in Journey's End would go to the federation, people who evacuated when the federation told them to would finally be able to return to their homes once the Cardassian colonists left.
Any worlds that the Cardassians acquired in a fashion similar to their conquest of Bajor would (hopefully) be freed from the Cardassians. Indigenous populations would see their worlds emptied of any Cardassian presence.
Stripping Cardassia of all it's non-indigenous population colonies, similar in a way to what happen to Germany following WWI, could happen as well.
In the end, the "Union" might consist of just the Cardassian home world, and the Cardassian home star system.