One may not kasher something on the festival itself, as it is considered rendering the utensil to become fit for use, which is not permitted on Shabbat and Yomtov. However, if the hotplate is on its highest heat from Shabbat into the Chag, this is sufficient for it to be considered kosher, as from meat to dairy a lower level of libun (‘firing’) is suitable. This is permissible when it is not noticeable that it is done for the purpose of koshering between meat and dairy.
Notwithstanding, it is desirable to put two layers of silver foil, cut before Shabbat and Yomtov, on the hotplate under the dairy food to be warmed up.