It is supposed to be automatic, however I have encountered this during the Yosemite upgrade. I'll explain how I fixed it.
For the sake of discussion, I'll use AUserName to refer to the user whose password is out of sync.
Generally speaking it's quite simple. Remove AUserName from the list of users that can unlock this disk, then add them back.
IMPORTANT, if you do not have multiple users who can unlock the disk, add one now, or have the recovery key handy.
Here's what you do:
- Click the Finder icon in the Dock
- Click Applications
- Double click Utilities
- Double click Terminal
In the terminal window, type the following:
sudo fdesetup remove -user AUserName
You'll be prompted to enter your password.
This removes AUserName from the list of unlock users. Next,
- Click the Preferences icon in the Dock
- Click the Security icon in preferences
- Click the FileVault tab
There should be a warning message that "Some users are not able to unlock the disk"
- Click the "Lock" icon at the bottom of the window and supply administrator credentials.
- Click the Enable Users button
- Click the Enable User next to AUserName
- Click Done