This is an interesting thread. Ultimately, per your conditions of carriage and the fare rules applicable to your flight, this does not appear to be allowed or follow standard IATA practice. If you read the fare rules for a W-Fare on SQ from AU-UK - the rules are pretty clear and follow a standard format. Change fee $70 plus possible service centre fee. The requirement to upgrade to a flex fare after departure makes no sense and does not follow how ticketing works on any carrier I have seen. If you had booked through full-service travel agent - they would basically follow the rules.

Now - these are the rules if you change your return flight after departure. Changing before departure can normally (but not 100% of the time) require a reprice using current fares (unless first coupon remains same - and in that case - even those require full reprice).

So if you are saying that after departure they are forcing a reprice to a completely different fare - even when other fares are available - then this is crazy - - but - what may be happening is that Amadeus is just blocking out all inventory for "you" . . . and this has happened to me a lot recently on LX - where on a return - trying to change or reprice - the system just would not allow me to get inventory that was clearly there. . . In that case - I walked down to the ticket office of LX and sat with them until they made it work . . even they could not explain it!