Congratulations on qualifying! Pre-Elite runs Thurs-Sun and has 8 rounds, just like Elite. The Pre-Elite (and Double-Digit) dogs generally run a slightly shorter/simplified version of the Elite dogs' course. What you might find helpful is to go to the Champs page on the NADAC web site, and scroll down to the bottom, where you will see links for the courses for all the rounds at last year's Championships. You can look at the Pre-Elite and Elite courses and see how they differ from one another and get an idea of what to expect. All dogs entered in the event do the first seven rounds, each of which is a 'regular' course but with a particular focus: one might have more jumps, or weaves, or tunnels, etc. Then, the top third, score-wise, of each group will progress to the finals round - ie. Sunday's second round.
The Pre-Champs classes on Tues and Wed will be classes just like you'd see at a regular trial and are not mandatory to attend - there will be a separate entry form for them appearing shortly I am sure. Some folks like to enter all of everything, some have work or other constraints and just do the Championships portion, etc. I would say that if it's your first Championships, it would be great if you are able to enter a couple pre-champs classes if you can - it's a nice opportunity for your dog (and you!) to get a little used to the venue.
I have only been a couple times but have found it really fun! Of course I was initially nervous as all get out ... but an experience that was still totally worth it and I hope I'm able to go this year. Good luck and I hope to see you there!