mr.a@razorcrest:~ % uname -ns
FreeBSD razorcrest
mr.a@razorcrest:~ % echo ${SHELL}
/bin/csh
mr.a@razorcrest:~ % sudo -k echo 'passwd:     files nis sss'>/tmp/nsswitch.conf  
mr.a@razorcrest:~ % sudo -k echo 'shadow:     files nis sss'>>/tmp/nsswitch.conf  
mr.a@razorcrest:~ % sudo -k cat /tmp/nsswitch.conf  
passwd:     files nis sss
shadow:     files nis sss
mr.a@razorcrest:~ % sudo -k -E sh -c 'export a=/tmp/nsswitch.conf; cp -fpv "${a}" "${a}".$(date +"%Y%m%d%H%M%S").baka;' sudo -k ex -sc 'g/^\([[:space:]]\)*\(passwd:\|shadow:\)/s/nis[[:space:]]//' -c w -c q /tmp/nsswitch.conf
‘/tmp/nsswitch.conf’ -> ‘/tmp/nsswitch.conf.20200520103634.baka’

I've a problem with 'sh -c' which doesn't seem to honor the '\(' character...

Works like a charm...

As suggested...