Thanks for linking that!
Yes, I understand why you would want it, but there are only two ways to do this:
- download, re-encrypt, upload
- store a key file with the real password
Option (1) is going to be super slow. You can use the Duplicati.CommandLine.RecoveryTool.exe to re-encrypt the files. The tool Duplicati.CommandLine.BackendTool.exe can upload/download for you.
Option (2) is mentioned in the link @mohak posted, and it has the disadvantage that you have a really important intermediate file, giving a single point of failure.