Originally Posted by
Yul_voyager
I think you are the only one they told that !
They said to me that the only change permitted without paying additional miles is changing Y for J on the same flight if J becomes available...
According to the T&C of AP, a reward can be changed without additional miles if there is no change of reward zone. This is just like the change of bookings made before the increase of miles requirement. They would just charge the change fee and can't adjust the miles.
This has to be true especially AP insists that this was not an error. This is a "special offer" promotion. So they can't ask for more miles when the reward is changed later.