ownership

I copied a bunch of preference files (those files and directories whose name starts with a dot) from my old RedHat home directory into my new Suse install. I had backed up the files being root and so all of them were owner:group root:root. In order to only change those files, that had root:root into jeltsch:users I executed from within my home directory:
sudo chown -R --from=root:root jeltsch:users .*

If you wanted to change all files on the system that belong to one user you could use:
sudo chown -R --from=username newowner /