Replacing valves is only done if there is damage to them such as bent or burned. Typically the valves and their seats are ground to restore a good seal. You should also have the guides checked for side play due to wear and replace the valve stem seals and stem to cam follower O rings for quiet operation. The head to block surface should be checked for warpage, the cam bearings, cam, followers and springs for wear. Finally, the cam to follower clearance is checked and corrected by changing shims as needed.