The way I understand it is that all the stories that are post ROTJ are moving to the Legends line and won't be apart of the official canon. However some of those stories and characters can, and in some cases, will most likely be transferred over to the official canon but not exactly as they were. So what does that mean? It means that characters like Thrawn and Mara Jade could appear somewhere down the line in the new official stories but the events and stories that they were in may be different. And maybe some will be exactly as they were in previous stories, it depends on how the new movies affect the canon. So we could see a resurrected Thrawn and Mara Jade in the future, which I would love to see again despite the fact I would be sad to see a lot of my favorite EU stories get erased from canon. I don't see them getting rid of Thrawn, especially after the official video had Timothy Zahn talking about the future of the EU. I hope. Right now everything before ROTJ has not been "officially" erased and is still canon, but can be thrown out if it contradicts the movies, TV shows, or any of recently announced novels coming out within the next few years. I don't see the stories about Plagueis, Bane, or Revan being greatly affected or anything before the Old Republic era, unless one of the spin-off films is set in any of those time periods.