acf/load_field only for NEW post

Hey,

I am using the acf/load_field field for an auto increment function.

This is working okay.

But how can I avoid to load this function when editing an old post?

At the moment it overwrites the existing value when opening the editor mask.

function fill_field( $field ) {
    $jobnr=get_option('pro_max-job-nummer')+1;
    $field['value']=$jobnr; 
    return $field;
}
add_filter('acf/prepare_field/name=job-nummer', 'fill_field');