1. ask Silly question - previously stored ("What is your dog's name?")
2. if OK, prompt to set new password - move existing to an 'old password' field somewhere
3. show warning at next logon for X weeks that password has been changed and that if they want to use their old password to enter it instead - Don't allow password change during this period.
4. if they enter old password, replace new with old, delete stored old and prompt for new security question (which has obviously been compromised)

Not 100%, but it will do...

cLive ;-)