Yes, if any/all of these options are changed for that plan you would need to re-upload the whole data set.

: Thanks a lot! I also wonder about that...
Based on your answer, does that mean that:
- All files will automatically be considered changed and automatically be re-uploaded?
- Do you need to run a consistency check?
- Will all previously uploaded files be removed from the backup set? (if not, is there a way to do so? in case you changed or added encryption?)

I did change encryption (before: none) on a backup task from the Service Provider Web GUI, but wasn't sure what to do next to make sure that ALL FILES on the backup media are encrypted. I understand that I can't re-encrypt all previously uploaded Data (I think!?), but would like to at least wipe out everything and then all newly uploaded data will be encrypted. Or since I'm using an "X days retention FROM BACKUP DATE", will all data be reuploaded over the X days when they change and then be "encrypted"? If so, what about unmodified files?

Some clarification regarding that:
When dealing with compression you would need to delete previously uploaded files on storage side manually and begin from scratch if you want your backups to be consistent regarding that. The same can be applied to the block-level option, but old backups will still be purged by your retention policy anyway, so in most cases you should be fine with that.

In case of encryption changes all of the previously backed up data will be overwritten upon the next backup plan.
Upon changing encryption settings you will see a pop-up window like on the attachment in this message. Clicking yes will re-upload all previously backed up data with new encryption options.

Retention policy doesn't really play any role in encryption settings, it only deletes the files according to your parameters.