I can't say I've had any issues changing flights or applying upgrades, even since the change to Sabre but I've generally been getting agents from I think the NZ call centre, assuming they have one.

I did have one drama with an agent when I wanted to change an Etihad award booking from Y to F; he wanted to charge me a cancellation fee etc but after some arguing I pointed out that their fare rules didn't explicitly state a fee for change of class, only routing or date of which I was doing neither.

So whilst he did have to cancel the reservation, refund me and then reserve me the new seat in F I managed to scoot around the change fees, which in my opinion was completely fair.