The ability to make this sort of change is a long standing pebble in the shoe of the OW alliance. IME about 50% of the time you will achieve your desired objective of getting the change made. If although you think you have made the change the flight still posts to the wrong account and you want to change it, you need to get it de-posted from the initial account and then posted to where you want it. A time consuming process involving lots of long calls with both airlines FF CS departments. With that said i have never been unable to get it done when i was willing to be persistent (eg when requalification or some other consideration was at stake.)