We consider changing both foil & cutter at the same time the best approach for maintaining your quality of shave.
Your cutting system works as a co-operation of the the foil & cutter cutting against each other. Changing just the foil or cutter on their own would be like sharpening just one side of a pair of scissors! If you have had your foil & cutter fitted for more than 18months we recommend this as an excellent way to improve your shave.