I had my Jetstar business flight swapped to an Emirates business flight, but not without issue.
Flew J-class AMS-SIN-MEL booked via KLM, SIN-MEL leg was operated by Jetstar and was cancelled, should have known better since looking at the flight history for JQ8 showed it was notorious for being cancelled.
Jetstar rebooked me on a flight the following morning on Emirates, however sneakily bumped me to the economy cabin with no communication on the downgrade.
I went to the KLM counter at SIN and asked for a move back to J class and an updated reservation, and the staff were happy to do so (I also have status on KLM so that probably also helped).
The next morning, armed with the updated eticket, I checked in at the Emirates counter, and they downgraded me back to economy to one of the worst seats on the plane, row 50 on 777-300! Even though the updated eticket was for J class! After pointing this out I was moved me back to J, but was a bit of a hassle.
To add insult to injury my baggage was also lost/delayed, same with many other passengers arriving at MEL on that same flight. At least we all arrived safely in one piece, and was lucky enough to experience Emirates business
Next time I’ll be looking for the KLM-Qantas codeshare flights since their partnership extends to frequent flyer benefits of both programs, and would be a much nicer experience!