is a copy of the C:\users\*** from W10 TP usable in W8 (copy-paste after clean install) on the same rig ?
No, you cant copy the whole thing But, aside the the data in documents desktops, photo which are pretty obvious, you can carefully move/copy the folders under appdata/roaming and appdata local of many applications (say, most of the email clienta, Vivaldi, Opera, Firefox, chromium, and so on) The same applies for /programdata Anyway if you have the whole windows.old folder you can just start with a live linux distro rt with the windows recovery consolle and move all your win 10 things in a windows.new folder then move al the folders inside windows.old back in the root directory. Your win 8 installation will start on the next boot. Then you have to move the documents from win 10 user to win 8 one.