The product description says the pillowcases are 20x30 which is a queen size pillow. Does buying king sets change the pillow cases to king size (20x36)?

Pillowcases 

It's possible that buying a king size set of pillowcases may change the pillowcases to a larger size, such as 20x36 inches, but it ultimately depends on the specific product and brand. 

It's best to check the product information or reach out to the manufacturer or seller to confirm the exact size of the pillowcases included in the king size set. They will be able to provide you with accurate information about the size of the pillowcases included in the set.