I recently concluded an extended period of time doing step without a step, for my knee like you. Now I am using a step with no risers and limit myself to 1-2x/week (the week of thanksgiving I tried step 3x and my knee complained for days).
Do whatever works for you! I love and was missing cathe step choreo which i why i did step with no step. One thing I will say, it certainly did not get my heart rate up the same way and my weight loss progress suffered for that. But for me it was modifying to all low impact AND no twisty/pivoty movements AND no step...