since alt is used to reverse the direction of the motion in commands like f and t, and shift is used to deal with extend (basicly selection) like in all other commands
I would like hiding that change behind an option to give people time to adapt to it, and a few versions afterward make the default of kakoune to use <a-u> for redo and U for undo selection.
and so u stays the same and <a-U> does the same thing (alt for redo the last edit, shift for selection rather than edits)
I apologise if that wasted your time, but I thought it was a somewhat better idea.