The exam these days is multiple choice. There is no assessment of the working. The only way CASA could change a mark is if they either changed the correct answer or increased the error margin and allowed for more than one correct answer.
It's improbable (but possible) CASA had the incorrect answer. The more likely answer, given the sudden increase in failures, it that they had reduced the margin and have now increased it. As the exam is marked by computer it is add to believe that it was a marking error.