For as long as I can remember, I have been running into an annoying issue with my iMac where I’d have to authenticate twice upon restart: once with my old account password to unlock FileVault, followed by my new account password to actually get into my account. I finally took the time to figure out how to fix this the other day. I couldn’t find a succinct example on The Google, so here you go. Note that this is for an APFS volume on High Sierra (10.13). If you are using HFS+ or an older version of macOS still, the instructions may be a bit different.
In Terminal run the following command:
sudo fdesetup list -extended
This will return a list of accounts that are associated with your Mac. The output should look something along the lines of below.
ESCROW UUID TYPE USER C20A4763-D4F8-4B3C-A889-0021B0735248 OS User justin 7DA69D0C-BA82-4575-AC68-CFE037778BA1 Unknown User 05E65DD7-AE83-4BD4-8B8B-AADA35A307E1 iCloud User
Copy the UUID for your specific user account as you will need it for the next command.
Once again in Terminal, type the following command:
diskutil apfs changePassphrase disk1s1 -user [UUID]
Replace UUID with your UUID you copied from above, obviously. You’ll be asked to enter your old passphrase, followed by the new one. After that, you should be able to sign into your Mac and unlock FileVault using the same password, once more.