I will suggest you to setup a isolated network, Dev& Test or Preprod and replicate all the applications and clients for testing the upgrade process. As you are in a better position to carry out the testing the application functionality and check the results.
Also you can review the functional levels of the DC and features of the DC - active-directory-functional-levels
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.