I guess it would depend on how old you are referring to. If you classify Rhythmic Step as old, then yes, I'd definitely agree with you, older = harder. However, if you are going back further than that, say with Power Max or Step Jam, then I think the choreography isn't as tough as the more current workouts.
I will say, however, that the cool down choreography used to be way more difficult than it is in the last bunch of series (which I am very grateful for! I never liked tricky cool down maneuvers after a workout is complete and I'm completely exhaused).
Shelbygirl