I saw you wrote:
Glass, plastic, glazed china, glazed dishes, Corelle, Pyrex, Corningware, or glazed earthenware do not need any Koshering, since they do not retain any flavor.
Does this mean I can use my year round china set (meat) for pesach (meat) as well without any koshering?
Correct
Does it make a difference if it was used for cold or hot things?
Yes for ashkenazim if it was used to cook chometz, you can’t use on Pesach. These are not normally cooked with
What if one didn’t cook on it but merely at hot chametz on the plate, can one use these things on Pesach?
Yes
Hi Rabbi. If a china-set of plates and bowls was used with hot chametz (but not otherwise cooked with/on), but then unused for about six years does that also help within Ashkenazi-custom for switching the china-set from non-Pesach to Pesach (without need to kasher)? Thank you Rabbi.
Yes if they’re glazed