You can sign up as soon as they start letting people in the park for breakfast reservations. The day I was there they let us in around 7:50. Even if they don’t let you in until 8:00 they are fully aware of your 8:00 AM reservation and will hold it for you. You can go straight and sign up for the Jedi training and then go right over to breakfast. It is nice because where you sign up is right by H&V. It only took us a few minutes to sign up.