The other option would be to give download users a credit for Cathe on demand so they can at least try a premix, and then decide if they want to subscribe on an ongoing basis (e.g. a credit equal to the amount the paid for the download).
What I mean is that you could create a version of On Demand that has all the premixes from the DVDs and allow download buyers to access the premixes that equate to the videos they have bought.