From what I read on the Lodestone, this entry stood out the most for me.

"As a note, gameplay will still be possible during the pre-migration download of the update files."

The wording seems to suggest you can still play the game with the old patch data BUT you would be downloading new patch information in the background to which on the 4th Sept, maintenance would ensure the new data completely replaces the old. Or so that's what I thought....