The discount certainly complicates things. To make it simple, oneworld have simply reduced their base prices by 10% - presumably the base prices will go up by 11% on April 12th.

Such changes require the original fare be used as a credit to the new fare, any difference is payable.

So methinks that if such a change is done before Apr12 then you will get the 'discounted' fare, after that the 'normal' fare.