Dumb question.

Nothing. When you swap Great Works, the swap is final. You traded what had been yours for what had been theirs, and you each now own the new stuff. Unlike trades of, e.g., Whales for Ivory, DOWing your trading partner does not unwind a Great Works trade.
 
In addition to that. If you are at war, and take the city that the Great Work is residing in, then you DO get the great work back, plus any other Great Works in that city.
 
To be sure, but in that case, when you take back what once was yours, you're taking what is now theirs. Has nothing to do with the trade (in fact, the AI may well trade to another civ whatever you traded to them, and you may take it back from THAT civ when you conquer one of their cities -- "Hey, didn't I trade that Great Work to Korea?").
 
No dumb questions that I can see. It is as Browd and Sixty4half have said- it's a clean trade so the status quo remains.
 
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