When a recurring event is changed to a different day, it seems that all previous instances are changed as well. This makes the older months calendar incorrect as a record of what has happened. Am I wrong or have I screwed up?

Welcome to the forum, jimbob.

The only way of having 1 event with 2 kinds of recurrence... is to have 2 events. If you have a recurrent event and want to change its behaviour, it will affect all the event from the start.
If what you want is to create some kind of split of the event, but keep the old one what I recommend to do is:

- e.g. I have an event every Monday since 08/27/2012. But since yesterday I need to change it to Friday.
- Edit the old event, and in the recurrence tab change "forever" to "until" and the date of 09-10-2012 or 09-16-2012 (17 not, because it will create a new instance).
- Then create a new event will the same summary and data, and the new recurrence.