The Users database table has an UpdatePassword column, I guess just setting it to true for all users would force all users to have to change their passwords on next login. That beeing said, I would test it first just to make sure my guess is right here...

Hello!  

There currently isn't an easy way to just force this on all user accounts on the site, but you can user-by-user.  However, this won't help.  I don't often put in SQL for people to run, for all kinds of reasons.  However, it's the best way to help you in this instance.  

The query below is a great start for what you're looking for.  You simply want to change the UpdatePassword column value in the Users table from 0 to 1 to trigger a password reset the next time someone attempts to log in.  

UPDATE [Users] 
SET [UpdatePassword] = 1 
WHERE IsSuperUser = 0;

The query above does not take anything into account, except for not updating any user account that is a superuser.  You may want to filter by the site (portal) using the UserPortals table, by security role (group) using the Roles table, etc.  

You can run this all from the SQL Console, without having to log into the database directly.  

Thanks everyone, that's what I thought.

Doing an update on the UpdatePassword = 1