It is not recommended to use the old potting soil in the new pot for repotting. It is because, if you are repotting a pothos plant after a year, then the nutrients in the soil is already depleted and it’s just a waste of space if you include it inside the new pot. On the other hand, if you are repotting your plant to a new plant within six months just for some aesthetic purpose, then you can use the old potting mix also.