I had heard the Qantas credits were extremely difficult to use, could only be used on full price fares and every obstruction that Qantas could put in your way was used to frustrate customers. I don't think all re-bookings could be done online? Don't know because I didn't hold any credits with them.
I did hold several Virgin credits, that were held in the money bank, can't remember if that's what it was called? Using them was easy, on any fair and could be 100% done online. From my perspective it couldn't have been easier and I didn't feel ripped off at any point.