Return TRUE if value has been changed and old value was not empty.

Return value

Bool

Full documentation

Prevent changing delivery_started value after it is given:

if ( $Order->delivery_started->isChangedOver() ){
    $Order->delivery_started->undoChange();
}

This is shorthand for:

if ( $Order->delivery_started->isChanged() && $Order->delivery_started->wasGiven() ){
    $Order->delivery_started->undoChange();
}