You do not have to reheat the pan with just half of it in it. I does not make to much sense, even for the freezing purpose. However, you can cook your lasagna in your 9x13 inches pan. Let it cool, and divided it in as many portions you want. Freeze them in freezing bags and bring out the portion or portions you want to cook into the fridge with 24 hours in advance. When reheating the portion (s) just put it on a foil paper, well wrapped, at 350 for aprox. 20 + minutes and eat it. Or you can reheat it in the microwave in a glass or microwave proof container for 4 minutes, depending on the power of your microwave and the size of the portion you are heating up. Does not make to much sense to reheat the big pan with half of it or even freeze the pan in the freezer. Freezers have very limited freezing capacity; therefore, freezing bags allow you to maximize space and move things more easily while adjusting your space around.