I didn't say "non-changeable", I said you can't change the ticket type, as in you can't re-book a light ticket into a flex or comfort flex one and a business basic ticket can only be re-booked into business basic again, not flex. I haven't had the chance to test this myself, since as of early 2020 I've never booked light again (and so far I haven't booked business light either), but it seems logical enough, that the two fare types that don't allow free changes also won't allow re-bookings to different types of tickets.