I have a One world RTW reward ticket and have commenced my journey. The highest class of travel is business class but due to availability a couple of my legs are premier economy. On one of those legs i saw a business class reward seat was now available so tried to switch. Qantas told me that because my journey had commenced the only thing i could change was time/date of flights. They said I’m not able to change the class of an existing segmentvor add new routing. Does this sound right? It sounds different to what the articles say online…
Hi @sallymcc and welcome to the community.
I’m not sure what the fine print is for a RTW reward booking, I know that with regular reward bookings you are not eligible to upgrade your seat - in the traditional sense of either paying extra money or with points (like you can when you purchase a cash fare) - unless it’s a QF flight number.
You can however, “replace” a segment/flight in a reward itinerary with a higher class segment if you have enough points etc… eg: Business class → First class. I’ve done this many times both prior to and after starting a trip - albeit not with a RTW reward booking
The only other reason I can think of that you might have been told this is not possible is that any change will affect the overall points cap that has been applied to your original itinerary - and if you have started the trip, it then becomes difficult to retrospectively apply a newly calculated points maximum to a past segment(s).
I’m sure there would be others in the forum here who could comment more authoritatively on the specifics of a RTW reward booking.
All the best 
I think the issue here is that they have already commenced their RTW journey. Is there any T&Cs regarding this part?