Function change_line 

pub fn change_line(
    path: &Path,
    place: usize,
    seen: &SeenLine,
    label: &str,
    amount: f64,
    date: Date,
    today: Date,
) -> Result<bool, String>

A line of the file changed in place: its label, amount and date; its links, its budget and the comment saying where it came from stay. A date to come makes it planned. The line is found as remove_line finds it; Ok(false) when it is not found as it was (seen): nothing is changed.