Based on this below it means you cannot make the 2 caption change until that opens up at approx. 10PM EST tonight (after Prelims). This is an update hostrauser makes.
PRELIMS: Thursday, August 8th; caption changes turned back on (roughly) 7pm Pacific / 8pm Mountain / 9pm Central / 10pm Eastern; 1 update, only 2 captions per corps
Then another 2 caption option follows after SEMIS.