Can I Force All Users to Change their Passwords?

You can expire all users passwords. See Setting Password Requirements and Expiring Passwords

"Selecting a password expiration takes effect immediately and applies retroactively. For example, if a site is not using password expiration then selects 90 days, all users who have not changed their password in 90 days will be forced to change their password on the next login."

You can force a one-time password reset using the Edit Multiple Users Simultaneously feature.