Hi, @Edwin van Ree!
The repository type can only be selected on a repository creation. However, a backup copy job is able to transfer backups to a repository of another type, and then old backups may be dismissed. This article describes the migration to an Incremental with Full repository (stores backups in separate files), but it works in the same way in the reverse direction:
https://helpcenter.nakivo.com/display/KB/How+to+Move+Backups+to+an+Incremental-with-Full-Backups+Repository
Let me know if I can do anything else for you.