Roof sheathing does not have to be replaced every time you need new shingles provided the sheathing is still in good shape. You will need to inspect the sheathing as the old shingles are removed. If there are large gaps between boards, holes in the sheathing, water or mold damage to the sheathing, etc., you might need to replace some or all of the sheathing. If the roofing material is properly installed and maintained, sheathing should last through several sets of shingles.