Datapoint.

I changed a USDM biz award open jaw BKK-HKG-AKL//WLG-SYD-PER-HKG-BKK into a simple return BKK-HKG-AKL//AKL-MEL-KUL-BKK 2 days ago. I managed this after many calls to US agents via teh Korean AA help-line (I'm in Seoul). Was aiming to 'straighten-out' the return from 42 hours!

Needed to pay the USD150 fee (this was expected), but did get a credit of USD55 in taxes, probably due to removing the transit at SYD or the long connection in PER. I had an improved route ex-WLG for the return as I thought it unlikely that the OJ could be changed without a full cancellation, redeposit of miles, and new award from AA. I didn't want to do that as I was concerned I'd lose my HKG-AKL on CX.

In any case, this seems to more than a simple change. And I am very pleased with the outcome.