I want to move content of a directory within another directory with the same folders.

Here's an example:

I have ./backup which has the directories base and test. Now, I want to move these directories to ./backupArchives.

I use this:

mv ./backup/* ./backupArchives

but I got the error

mv: cannot move './backup/base' to './backupsArchive/base': Directory not empty

I tried using --force, but no luck. Is there a way to move it when folders already exists?

Note: I just want to merge contents, there's no overwriting.