Under-tank heating pads can be moderately warm or extremely hot, it depends on the make and manufacturer. It's best to use them outside the tank rather than in, esp. with lizards, and a thermostat would be a good idea to ensure the temp stays constant and the appliance doesn't overheat.
What bulb do you use at present? Does it stay on all the time, or just during the day? What wattage is it? Perhaps you could measure the temps in the tank when you move it into the new room, before putting the heat pad in, to make sure you will need it. It may stay warm enough?
