Changes to Assessment Criteria after assessment has finished

So a module on the course has decided to strike a component of an exam that we had taken worth 50% of the final mark and decided just to assess us on the other 50% of the other component instead. Does anyone know if this is even allowed?

It just seems highly unfair as some of us would have put more emphasis into the 50% of the exam which has been discredited than others and we were only told about this change after the exam had finished.

Has this happened to anyone else at Uni?