Thanks Leonie
So I did that. Download Exiftool (in fact I've had already done it), give up on GUI tools that do not seem to exist for that purpose on Mac, move my entire pic directory , then launch 3 cmd line in Terminal (that do work in batch):
exiftool -overwrite_original_in_place -preserve "-Make=Canon" /mydirectorypath
exiftool -overwrite_original_in_place -preserve "-Model=Canon EOS 100" /mydirectorypath
exiftool -overwrite_original_in_place -preserve "-Lens=Canon EF 28-105mm f/3.5-4.5 USM" /mydirectorypath
then consolidate database.
I am now confronted with something strange:
All pictures have been modified (as stated by exiftool and checked with Preview: the original exif has been modified) but in Aperture, for an unknown reason only a third of them appear to be.
It seems that the finder did record a modification date on the files only on nearly a third of them, and these are the ones that Aperture sees as modified