AnandTech suggests this will be done at the expense of NAND flash memory capacity.
In fact, based on claims made by Intel’s CEO, it looks like the company is more than inclined to ramp up 3D XPoint/Optane memory at Fab 68 at the expense of 3D NAND volumes. Given the scale of the plant, using it for Optane would certainly help to drive costs of persistent memory down. However, it is hard to predict when and if Optane volumes could have an impact of 3D NAND volumes given all the differences between the two types of memory that span from process technologies to costs to applications.