Can [Update Needed] be removed without disabling/enabling FileVault? I have an additional user [Update Needed] appear on login screen at every boot, which doesn't affect usability but a bit annoying. I googled for solution, but all of them are just disabling and enabling FileVault again. That would be time-consuming, somewhat harmful to the SSD, and would leave bits of sensitive data unencrypted on the disk. Is there any other way? Like some terminal commands?
If the Mac in question is running OS X 10.8.x or later, you can remove users from the list of FileVault enabled accounts using the fdesetup remove command. To remove the account using the username, run the following command:

sudo fdesetup remove -user username


Successful removal of the account will not produce any additional output. 



If the account being removed is not currently enabled for use with FileVault 2, an error message will be displayed.



Does the icon you're seeing look like this?



If it does, this isn't a user account. It's a password directly associated with the disk's encryption. You will need to decrypt in order to get rid of it.