@BetterRed
thanks for the inspiration to use calibredb and to to it on a small subset of comics !
I did not use the command add, but I renamed all cbr files to cbz and then ran the command "calibredb restore_database -r"
a new database was built using the same structure with the right formats
it took some time but it's running perfectly in the background !
I consider my problem finally solved - all my comics are in cbz format in the calibre database without having to introduce them back one by one !