Changing a Sales Order that has been partially invoiced?

SUGGESTED

We are using version 2019. According to an enhancement sheet for various versions we are supposed to be able to add, delete, or edit lines on sales orders that have been partially invoiced since version 2010.

It appears we cannot do this. All options for any changes (except canceling an item or editing comment lines) are grayed out. What am I missing? Perhaps an option needs to be enabled to allow this? Was this option removed in later versions?

It would be very useful for us to be able to do this.