Pre order will be Friday. They won’t change that. They already have strain on the servers right after the event with release candidates of software being available. Also if preorders go live immediately after event it will make it difficult for the likes of people at the event to preorder. They don’t all get pre release devices to take home with them! Suppose Apple could give them a special link!
They already open pre orders for Apple Watch straight after the event so not sure why it would be any different for iPhone soon.