You may want to make a plan/package setting to all plans available on a server and can be done using our "whmapi1 editpkg" API.
The API documentation can be found at: Update hosting plan - whmapi1 editpkg
Let's say you want to change MAX_EMAIL_PER_HOUR to 500. The following one line would make that change to all packages/plans.
ls /var/cpanel/packages/ |grep -v extensions | while read x; do whmapi1 editpkg name=$x MAX_EMAIL_PER_HOUR=500; done