Thank you very much for your help. I have tried your suggestion, which did not seem to have an effect (and I was a bit afraid of losing some of the student assessment grades, but that did not happen. Maybe it could be indicated a bit more clearly).
However, I'm happy to report that I have been able to resolve the issue by changing the "Grade for submission" in the "Grading settings" of the workshop. That has the effect that all submission grades are (re-)scaled according to the this new maximal number of points, and apparently the new weights are taken into account as well.