You can technically put new roof sheathing over old, but it isn’t recommended. Firstly, it isn’t hard to remove sheathing. Secondly, when you install new over old, you trap whatever moisture is in the older layer. This can cause problems later on, especially if you have a humid house. Another reason not to add new over old is the weight factor. Finally, when you cover an old layer, you lose the ability to inspect it for damage, rot, or mold.