Shoutbox

Yes, you can re-encrypted them with another key.

Open the plus settings windows by typing '/prefs' in a conversation window. (don't worry, your contact wont know.)

Then go to the Conversations tab, then logs encryption on the left.

You should see 'Re-Encryption', from there you can change the password of the files and above that you can choose a new password for new files that are created.

Good Luck.