I've been instructing for almost two years, and so far I haven't had to modify my lesson plans much. I put a lot of detail into them originally in college. That paid off. I do change things once in a while if I think of a better way to teach something. Everything in the binder is on my computer as a Word file, so it's easy to modify. I have lots of notes.
My most common additions are web sites, such as the online A/FD, NASA ASRS forms, and 8710s. As more things become available "paperless," I can see I'll need to add even more web sites. My students like to know the exact site rather than do a search, it seems. So I keep the addresses handy.
Another thing I've had to add was info on sport pilot training. It's not really a lesson plan, but so many people have asked me about it that I needed to make myself an outline.
I still use my lesson plans frequently as a way to stay organized. I've got most of the info in my head, but I like to refer to my notes to stay on track. As for "case studies," I've got a nice collection of printed weather info/graphics from around the country, some accident reports, pages from various POHs, etc. It's helpful to give a variety of real-world examples beyond the airplane you're flying and the airport you train from.