In some cases it is not possible to change the scale in the form. It is reported, for example, in
https://moodle.org/mod/forum/discuss.php?d=433826

http://www.halmaclean.co.uk/moodle-competencies-moodle-grading-scales-when-it-all-goes-wrong/
Then the error “errorscaleconfiguration” is always displayed:
"The scale needs to be configured by selecting default and proficient items."

Only a switch of the scaleid in the record of mdl_competency_framework helps in this case