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After Dirge’s ability resolves, does the bonus change if the number of characters in my opponent’s KO area changes later in the turn?

No. you count the number of characters in your opponent’s KO area when the ability resolves. After that, even if you KO another character, the attack bonus won’t change.
Source: Wizards of the Coast (2019-03-01)    【Link to Ruling】
Tags: Dirge, Doombringer