You can use --dry-run to see what happens. Copy makes a destination like the source without deleting. Sync makes the destination look like the source with deleting.
my q regarding rclone?
I'm not skilled in unix and NAS OS, so I struggle with understanding the "path" syntax.
For DOS i understand perfectly the idea of copy c:\Private n:\Backup\Private
I hoped to get specification how to make the exact syntax (not like the help text source: destination:)
I don't use rclone as a backup tool myself as I don't think it really is one (just my opinion) but backup-dir might be something you want to take a look at depending on what you want to happen.
That is a limitation in the current implementation: Any devices other than the default "Jotta" device are effectively read-only (edit: not strictly read-only perhaps, you may be able to create directories etc, but at least uploading files is blocked). Not very explicitely documented, unfortunately (will see if I can add a sentence about that). The limitation could in principle be removed quite easily (famous last words...), but there are some differences and possible pitfalls to consider when using other devices, so it will need a dev to invest some time to make sure these are properly handled. See the following for some related discussion:
Regarding your use case: I assume you have been using the official Jottacloud application and its backup function so far. Do you plan to switch entirely to rclone, or continue with both? If switching to Rclone: You could move the contents from "Backed up" into "Archive" where it is fully accessible from rclone. Don't know if this is possible to do from official client or web gui, but rclone does support Jottacloud's built-in deduplication feature, so if you have room for it in your account, you could just copy again from source into Jotta/Archive and it should go very fast if it is mostly duplicates of what is already in "Backed up". Then later at some point when you are confident you have all in Archive, you can delete the contents from "Backed up".