I just wanted to second the suggestion to not use the step in order to go easier on your knees. I have not been using a step for the past 4 to 5 weeks, and I'm really noticing a difference.
I have also been keeping my workouts almost entirely low impact. I have found it pretty easy to modify Stepworks, PowerMax, some of Stepfit, and Allstep and Step and Intervals (both from the CTX series). I have found it more difficult to modify cardiokicks and CircuitMax. However, if you put some time into it, I believe any of cathe's tapes can be modified to low impact (as long as you are not using the step. I haven't tried it while actually using a step.)
Sometimes it is frustrating to have to come up with a modification, but modifying Cathe to low is still so much better than suffering through a low impact workout made by most anyone else! (No offense meant to other instructors--please, if you know of an intense, well instructed low impact tape, I'd love to know about it!!)
Wendy