Rhea G,
If I undertand your question, I think you can have a consistent rotation but variety in the DVDs you use as long as they are the same type.
For example, if you are doing total body weight training, use muscle endurance or muscle max or any other total body workout that is similar. That way you're not doing the exact same workout for 4+ weeks.
Right now I'm working out 5x/week alternating cardio(mostly step) and total body. I have 3 total body workouts and a lot of step DVDs to mix into the rotation.