Changing of the Syllabus

Thinking about yesterday's discussion of differences in teaching Science vs. Humanities topics, I wondered how often we professors, over time, change the syllabus of the courses we teach, and, if they do change, why they change. By the phrase "over time", I am trying to say that I mean changes over the years, not within a particular term.

I think it is entirely possible, in any discipline, to teach a "classic" topic that doesn't change dramatically from year to year, or even decade to decade, no matter how much the broader field itself has changed. And then there are courses on "topical" topics, that include or focus on concepts or methods that were unknown in the past, perhaps even in the fairly recent past; this, too, could apply to any discipline, even if not to every course within every discipline.

I was thinking about some of my "classic" science courses in this context, and realized that they have actually changed quite a it over the years, not in terms of the core concepts, some of which have remained unchanged for a long, long time, but in the examples I discuss. There are new applications that did not exist a decade ago (or less), and if I taught the course the same way that I did 20 years ago, students would learn the basics but not be aware of how these can be used to do Modern Science.

I don't think I could specify exactly how much these courses have changed with time, but a very rough guess would be somewhere between 20-40%. In some cases, these changes are reflected in the syllabus (in terms of the list of course topics) and in some cases, not so much. That is, the listed topics are the same, but what I say about them has changed a lot over time. I refer here to changes that are driven by advances in the field, not variations I might introduce to keep from being bored with teaching the exact same thing year after year.

The above discussion refers primarily to undergraduate courses and to some advanced courses, but, for me, graduate seminars are a different beast. The topics of graduate seminars that I teach change significantly over time. It would be impossible (or, at least, very unwise) for me to teach the same graduate seminar over the years; things (including the scientific literature) change too much, too fast for that. I could organize a seminar on the same or similar (very) general topic every 10 years (for example), but the content of the seminar would be very different each time. I would think that this, too, could be said for many other disciplines.

Have you ever taught a course that literally could stay pretty much the same year after year, for a long time, perhaps even decades? Or, (more likely, I think) if you teach a course with content that can, should, and does change with time owing to advances in your field, how much does it change (per whatever unit of time is relevant to your situation)?