Only P,U,T fares under GoLight are non-upgradeable. That means that every flex and saver fare should be upgradeable regardless of the booking class.
Also, the A3 website lets you easily verify the booking class: in the search results screen, once you have selected a flight, you can click on the arrow appearing at the right part of the screen, next to "flights" and the booking class will show.
Finally, you can always go via the call center to select a specific class, but for every reservation made that way there is a 20 euro fee.
Edit: Apparently
gpb_croppers63 was faster
