Time Machine - Encrypted Backups - How to Have TM Prompt User For Password Again
I am not sure, but this may work. Try this:


Open Time Machine from System Preferences 
Turn Time Machine Off
Select Backup Disk and turn Time Machine back on
Choose to Encrypt Back Up
Re-enter your password, or create a new password
Wait for it to encrypt
Turn Time Machine off, and then on again to test it out