Originally Posted by cas_de

Yes, but this approach has mainly been used when the booking class did determine the level of flexibility (T -> super restricted; Y -> flex). With the introduction of the LGT/CLS/FLX fares this approach became a lot less useful, because worst case you could want to upgrade from a T_CLS_ fare to a T_FLX fare. This way you would only safe the rebooking fee, but a fare difference does still apply. Unless you want to take care of the same day changes that you are allowed to make when travelling on a FLX fare.