The way the plugin knows to force someone to change their password is if their last visit date is: 0000-00-00 00:00:00
If you want to force every user in your system to update their password, you could modify all their last visit dates to zero and therefore everyone would have to change their password. This SQL should do it:
UPDATE jos_users SET lastvisitDate = '0000-00-00 00:00:00'
I would highly suggest doing this on your development site first to make sure it doesn't cause issues with any of your other extensions or anything.
If you would rather change the plugin to fire using some other criteria, I can help with that as well. Just let me know what criteria you want to use. Since you will be making this change on your own site and this doesn't have to work for everyone, we could even just keep a temporary table to know if users have already changed their passwords.
Let me know.