For plans where there is no deviation in planning expected or wanted, for audited changes, it is mandatory that a plan that has been presented to Executors will not change during execution.
Such a plan once started cannot be changed. Only changes are in Executors that can be reassigned.
For plans where changes during run are standard required feature, this has to be checked during creation of plan (see picture below). Changes such as adding or removing tasks at long term plans are common.
Full set of icons to add/edit/delete Task is available: