It's been opening as much as 3 hours early (e.g., 6am with a scheduled 9am opening), with people able to enter the park and get a RoTR boarding group assignment. Other attractions (think rides, not shows or characters) have been up and running at that time, as well. As others said, RoTR boarding groups have been filling up before the park's official "opening" time many days. If it were I, I'd plan on arriving 2-3 hours before the park is scheduled to open (with a good book and a snack in my park bag, to keep me busy in case I have to wait to enter the park).
However, keep in mind that the situation is constantly evolving, and the operational procedures seem to be changing by the day. You'll want to keep stalking these forums and similar sites right up until your visit.