If anyone wondering why it is call 'downcoding', it's because there's WHO ICD coding standard or international classification of diseases now at version 11 or ICD-11 [1]. It's mainly used for classification of disease mortality not morbidity, not until the latest version iteration of ICD-11 in which it now caters for both [2].
Due the usefulness of the diseases classification coding based on ICD, it's also being used in many part of the world especially in US for healthcare insurance claim purposes.
[1] International Classification of Diseases 11th Revision: The global standard for diagnostic health information:
[2] ICD-11 vs. ICD-10 - a review of updates and novelties introduced in the latest version of the WHO International Classification of Diseases: