They seem to default to late dining when booking online. It’s not uncommon that the option to switch to main (early) dining is not available at the time you book, but you can request to be put on the wait list for main dining. On the few occasions where we have not booked on opening day, we’ve had to do that. We’ve been fortunate that the wait list has always come through, but some report having to wait until onboard to request a change if the wait list did not come through.