I hate to reply to my own post, but I’ve at least figured out how to make iTunes “forget” a password in iTunes/ Windows:
1) Shutdown iTunes.
2) Make a backup copy of %USERPROFILE%Application DataApple ComputeriTunesiTunes.pref.
3) Using a text editor (WordPad worked for me), remove the line in the above file that starts with “Keychain=”.
4) Save, and restart iTunes.
This will remove *all* stored passwords. It may have other side effects as well — use at your own risk.