how to change shortcut to any key I want?

I use "U" and "R" for undo and redo in most of my softwares, the keys are closer and easlier to use, when I try to change undo shortcut to "U" in photoshop it doesn't allow me, Is there a way I can do to force the program to allow me to do that? if not It would be great if you make that possible in the next updates