I agree with Anthony C. No need to change anything. I used the same ticket at The MET at the Cloisters. Realistically, you'll need one day each at each museum. There's just so much to see...to appreciate...not enough time to take it all in.
You can use the same ticket on both locations on the same day, so no need to change anything.
Call the met and do the same reservation at the met maybe its available
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