Currently booked on flights TUN-DXB, DXB-BKK (stopover) then BKK - SYD

Originally an economy flex+ outbound, business flex return, however I have upgraded all outbound segments to J with miles.

TUN-DXB, DXB-BKK and BKK-SYD are all showing D available and the first and last segment are showing Z available (DXB-BKK is a 2 class). I was hoping to cancel my miles, purchase D and then upgrade with miles to F. However I have called and the agent says they can only price me a business flex outbound?

J is wide open on all the flights so chances of an Op-up is 0.

Any suggestions?