I'm currently middle of DONE5 and trying to reroute it to make it DONE6 and only pay the historical fare difference. However, AY is pricing it so that I would need to pay difference with current exMPM fare and paid fare because of the late ticketing date.

The original ticket was booked, prised and deposit paid with cheap exMPM prices from QF on 3.8.2016 when QF had old exMPM fares still filed in the system. The final payment and ticketing took place on 6.8.2016. At that time QF didn't have anymore the old MPM price, only AY/JJ/LA/MH/S7/UL had that price left at the time of ticketing.

The ticket is currently on AY stock. AY first said that LA should make the change, but LA is not agreeing to touch the ticket for anything. AY finally agreed to do the change, but they are quoting the price difference between old and new exMPM price.

Should I be able to upfare my ticket from DONE5 to DONE6 by paying only the old fare difference (and tax difference of course)? In my understanding it should be possible given that AY had the price filed at the time of ticketing. And how should I proceed to get the rerouting done?