That is an out of the box control so as I stated it should have been upgraded for you. Did you perform any cutomizations to it? If so, there should be a Role_Edit_Users.aspx.cs.new file in the same directory. There you should be able to compare which lines are different.
Did your upgrade complete successfully or did you have errors? My concern is if you're modifying the out of the box controls, your upgrade probably failed and you're going to have a lot more problems than this one if you have to modify 7000-10000 files.
As I stated, you might start with a fresh install of v8 just to compare and nothing more. You can easily do this on your local machine with the Kentico Installation Manager.