I needed to change specific owners and groups for files that I copied (with tar cf) from one server to another with (some) different owners and groups. I used the syntax below:

chown -R --from=:currentgroup newuser:newgroup /some/directory 
chown -R --from=:currentgroup :newgroup /some/directory 

The solution, as so often, came from the Stack Exchange Network:

https://superuser.com/questions/312616/how-can-i-change-all-files-belonging-to-one-user-to-another-user