shabbat water heater

Question

a) if one forgot to turn down the hot water heater to below 125 before shabbat, can you turn it down on shabbat? If the fire is on you will be shutting it off. if the fire is off you will just be delaying it from going back on. do you have to make sure the fire is off? b)if you turn the hot water heater down 10 minutes before shabbat, and you use hot water 10 minutes after shabbat starts then the water in the heater is still hotter than 125. when the new water comes into the heater it may get cooked to above 125. is this a problem? should I make sure to turn the temp down from friday morning?