With this command:
find /starting/path -user "original_owner" -exec chown someuser:somegroup {} +
You can omit ":somegroup" if you don't want to change the group of the files/folderFor example to change from root to foo of foogroup in the folder /var/lol use the command:
find /var/lol -user "root" -exec chown foo:foogroup {} +
NOTE: You must be root to change the ownership of a root-owned file!
Source
superuser