Changing the password of a file vault 2 protected disk is fairly easy:

At first fire up the terminal and execute diskutil corestorage info /Volume/DriveWhichNeedsaNewPassword

If you are key-stroke aware you can abbreviate corestorage with cs.

fabian@heartofgold ~ diskutil corestorage info /Volumes/Time\ Machine
Core Storage Properties:
   Role:                       Logical Volume (LV)
   UUID:                       111DA74D-EDC4-498B-AE2C-50E07ACAE7CA
   Parent LVF UUID:            34B66F83-1680-493E-AEDF-1409F51A898A
   Parent LVG UUID:            718CDEB9-01F1-4FFA-BB1B-FE1EE92F9881
   Device Identifier:          disk3
   LV Status:                  Online
   Conversion Status:          Complete
   Content Hint:               Apple_HFS
   LV Name:                    Time Machine
   Volume Name:                Time Machine
   LV Size:                    499445116928 B

Copy the UUID (in this case this would be 111DA74D-EDC4-498B-AE2C-50E07ACAE7CA) of the drive and run disktuil corestorage changeVolumePassPhrase theUUID. Enter the old password, then twice the new one and after a few seconds you are done

fabian@heartofgold ~ diskutil corestorage changeVolumePassphrase 111DA74D-EDC4-498B-AE2C-50E07ACAE7CA
Old passphrase:
New passphrase:
Repeat new passphrase:
Started CoreStorage operation
Logical Volume passphrase has been changed
Core Storage LV UUID: 111DA74D-EDC4-498B-AE2C-50E07ACAE7CA
Finished CoreStorage operation