If I save an experiment using Igor Pro 7 and later and open it in Igor Pro 6, the bullet characters in the history window are replaced by • (on Windows) or ‚Ä¢ (on OS X). Why is this?

This is an expected consequence of Igor Pro 7 and later using Unicode (UTF-8), and Igor Pro 6 using system text encoding. I'm afraid that this is something we will have to live with.

There are many other consequences of switching to UTF-8. If you're interested, execute this for the details:

DisplayHelpTopic "Text Encodings"