Welcome to my hell. I had the exact same problem a few months ago when I tried to change it myself online..
I chatted with support and the same thing happened to them, they'd change it and it would revert back. Then I was told that only phone support could fix it, but I gave up after calling them multiple times 6 times and just getting silence on the line.
In the end I managed to get it changed through the My Optus app on my first attempt.