There will be a preview period, just like there was for IoA. It will be open to Employees and workers involved in the construction of Epic. IoA did it. Animal Kingdom did it. It happens for every major “dry-park” expansion in the world. It confirms operational capabilities AFTER confirming rides work.
MAYBE AP holders will get a preview week they can reserve.
Water parks, because of their smaller scale and need for actual people to test the rides themselves, have to just go for it. Wet parks rarely do extended previews because they don’t really draw enough people to be worth wild. Water parks have to max attendance.
That said, the first 3-6 months for Epic will likely be blacked out for most AP holders. Expect it.