Gmar Chasima Tova.
Parve soup heated in fleishig (ben yomo) pot. Next day heated soup (covered) in milchig microwave. Should microwave be kashered to be used for milchig?
There is no need to kasher the microwave, since the soup was not really fleishig, rather only nat bar nat of fleishig. The taste of the meat went from the meat to the pot and then from the pot to the pareve soup. Then from the pareve soup to the microwave. Taste of meat that traveled three times is not strong enough to cause the microwave to need koshering.
Shach Y”D 94-15.