Reb Shlomo Zalman Auerbach allowed one to change the setting on a Shabbos clock to keep an appliance that is currently off from turning on, because it does not cause a fire to be extinguished just never to light. However many Poskim disagree. Additionally, Reb Moshe Feinstein was concerned that moving the pins is muktza. So while there are grounds to be lenient, it is advisable not to.
However a couple of side points. Where to you keep your cholent overnight if the platta is off? Also, what are you putting on the platta when it comes on in the morning and how? Wet, cold foods may not be placed on the platta at all, and even dry foods may only be placed on an intervening pan according to most opinions.