I finally found a tool to convert cbr files to cbz files within the file and directory structure of calibre using Izarc (select directorries to convert, select all cbr files, first rename to rar, convert rar to zip, rename the zip files to cbz)
After that I have have both cbr and cbz files in the same directory in the callibre library
If I now remove the cbr file in explorer and I perform a database control, then it is mentioned that one format is missing and one format is present but not in the database.
This can be corrected but I do not see any option to incorporate a lot of cbr files into the database without going over a manual addition of a format.
Is this not possible automatically (not removing the extra format but effective adding it)