What issues may be at play when adjusting the temperature of an urn on Shabbat, e.g. from 200° F to 80° F, specifically on an urn pictured below*? (Assuming the water is already pre-boiled before Shabbat and no lights will turn on when temperature is adjusted. Would also appreciate technical input; it greatly helps in determining halachic outcome.)
*One company I am aware of that makes this model is Magic Mill.