The files uploaded are encrypted with the passphrase you choose. Because of this, changing the passphrase will mean that:
- you cannot read existing files
- new files will be encrypted with a different passphrase
In other words, you would need two passphrases to restore the backup. For this reason Duplicati actively prevents you from changing the passphrase.
In some very special cases you may want to use another passphrase than what the database has recorded (the passphrase is not stored in the database, only a hashed value).
Most likely you will never need this option.