Oh OK. I can give some answers to those concerns if you wanted. If not, never mind.
1) The encryption options can be changed. However, if you login to the online vault on the Online Backup Settings page, they can't be changed. It will say above them "Encryption details cannot be changed while Online Backup is enabled. Please Logout". If you then logout on the Online Backup Settings page, they can then be edited. You can then login again on Online Backup Settings - as long as no data exists on our servers. If data still exists on our servers it will report an encryption mismatch - the data on our server will need to be deleted before it will let you login with a different key.
The reason for this is that once you start backing up to our vault, the data on our vault is encrypted using your key. We then only backup changed files. If you could change the key it would cause a big head ache as new files would be encrypted with new key, but the other files would be with an old key. To download a snapshot you then need to know both keys! So we lock encryption. If the key needs changing data needs to be removed from our vault. This can be done by logging in and requesting deletion for the blog in question - this then starts a 24 hour grace period in which you can cancel, after which everything is purged.
One thing you might notice is that if you are using online backup, you can't choose different encryption key for say, a download backup, or an email backup. The key is locked and must be used for all three. You can't also do an unencrypted download backup. This is something we're improving soon so you can use different encryption per backup or turn it off per backup (not for online though of course.)
2) In this instance you would Logout on Online Backup Settings. Then login at wordpress.backup-technology.com to delete all data. A day later once it is wiped you can then change encryption on General Settings and then log in again on Online Backup Settings - it will start using the new details.
If you were using the exact same password to decrypt that is in the General Settings page and getting decryption errors, it might be due to a corrupt file that you are trying to decrypt. If you want to try again. Make sure the file downloaded fully. We are aware of backup files for download sometimes not correctly downloading from some hosts if they take more than a few minutes to download. The web browser loses connection due to a script timeout on the server and assumes the backup to be complete. So the backup page says "The backup size is 100 MiB" but your web browser downloads a file that is only 85 MiB. We're working on a fix at the moment to be released in a few weeks which changes the way we download large backup files.
Hope this addresses any concerns. If you did want to try again we can help with any issues to get them resolved.