Not sure how big of a market you are talking about here, nor the material(s) involved - for example, is it a straight textbook or are there lessons, syllabi(?), etc
Create a free version and a paid version. The paid version should obviously provide some substantial improvement and/or additional features from the free version. Perhaps those lessons and/or syllabi
If this is an on-going process of improvement then at some point you could make your older versions free and charge for the newer version -- even have some sort of a step down. Say full price for v3.0, half price for v2.0 and free for v1.0
I mean, thats how software has done it for years and that seems to be sticking around so it must work, right?