Like everyone else, it's the comment that two characters will die who she hadn't expected to die, while one she expected to die got a reprieve. She followed this up by saying that the final chapter now has "minor edits."
I think there are certain classes of character who are "immune" from the changes because of the comment that the changes were minor, and that they were, in fact, changes. That's not saying whether any of the "immune" live or die, just that they aren't likely to be the ones who changed.
The first category of immunes would be the major characters--Harry, Ron, Hermione, Dumbledore, Snape, Voldemort. Those are people whose fates would have been most covered in the original epilogue, and changes in whether they live or die would be major changes, not minor ones. If Dumbledore comes back to life as a jester in Queen Hermione's court, we can safely assume that wasn't one of these changes. It would be weird, but it would definitely not qualify as "minor" change from Dumbledore remaining dead. Ginny is not as major a character, but she was set up as important to Harry, and therefore was most likely covered.
The second category would be the extremely minor characters--Lavender Brown, Parvati and Padma, Aunt Marge, Stan Shunpike, Oliver Wood, etc. These probably appeared in JKR's head as she wrote, and weren't in the mythical epilogue.
The third category would be latecomers who don't really have a series-wide arc, for the same reason. I'd count out anyone who was introduced after the mid-point--Tonks, Kingsley, Slughorn, Luna, Greyback, and so on. They may introduce a few minor edits of their own in introducing them into the epilogue, but their fates probably weren't there in the first place to have "minor changes" made to it.
There are others like Uncle Vernon who were always there, but probably don't have enough weight (ha-ha) to account for major changs in their arcs.
That leaves a pretty small group of characters who were there from the beginning (or close to the beginning) and would have had their fates mentioned--most likely briefly--in the epilogue. Remus Lupin, Minerva McGonagall, Draco Malfoy, Neville Longbottom, Petunia Dursley, the Weasleys other than Ron and Ginny, Viktor, Fleur. I'm probably forgetting a few.
Candidates for a reprieve
For this, I'm trying to think of characters who had a likely death arc, but have had their arcs change direction.
Remus Lupin. Remus is the most obvious choice. When he was introduced, it was in the context of the dying past generation, and he was surrounded by death. He had no apparent future. Two of the other Marauders are dead. Harry's protectors die. But Remus was pulled back from his position as one of Harry's protectors and put on a level with a friend, and he was given a love interest, which connects him with a symbolic future. It's quite possible that in the original epilogue, there was a line like, "They visited Lupin's grave together," which could be changed with a minor edit like, "They visited Lupin and Tonks together."
Neville Longbottom. Neville started his career as a semi-hero by standing up to Harry, Ron, and Hermione and getting petrified for his troubles. He developed into a tragic character. But in OotP, he began to develop his skills and became less of a bumbler. In HBP, he's developed a friendship with the new character Luna, which, though not a romance, is a secure base for him. A reference to poor, lost Neville could easily be changed with a minor edit to a reference to Neville just moving away.
Percy Weasley. For the opposite reason, Percy may have gotten a reprieve. He started out on a kind of annoying-but-good track, but over the last two books has gotten increasingly odious. His death wouldn't tug enough heartstrings, and therefore might have been pulled out.
Charlie Weasley. On an oddball reason, I think that maybe Charlie was originally supposed to have bigger role than he ended up having--he was mentioned repeatedly in PS/SS, and I get the impression he was supposed to be Ron's favorite brother. Excellent choice for a death--affects the characters a lot, but doesn't necessarily kill the reader. But Charlie didn't end up playing much of a part, and it would be hard to sell a lot of grief for him.
Characters who may have unexpectedly passed away
Neville Longbottom. Yeah, I have him on both possible lists. He may have originally been meant to play a large part in the action, but because he slipped into the background, he becomes a loved character whose death could mobilize the others.
Minerva McGonagall. She was set up to take over for Dumbledore, and was largely seen in terms of her functionality. In the past two books, she's been active and vital, and has proved herself as one of Harry's protectors, at least in his battle with Umbridge. She's a minor enough character that JKR might have originally just mentioned that she was now Headmistress of Hogwarts, but now mentions that she is dead and someone else is in charge of the school.
Draco Malfoy. Up until book 6, Draco was a one-dimensional jerk on a path to becoming a Death Eater. It's quite possible that his original fate was put down as "Draco Malfoy was rotting in Azkaban." But as of book 6, Draco's family relationships and conflicts are introduced, and he's put into a delicate moral position that could well lead to his death, and the single line about Azkaban could be changed to a single line about not judging too quickly.
A Weasley other than Percy. As many people have pointed out, Molly's boggart is definitely setting up someone, and there are too many Weasleys to have everyone get th rough it open. If Percy got a reprieve, it's possible that one of the other brothers took over the role of Weasley martyr. Bill would be my first guess, though if there were two unexpected deaths, it's possible that it's the twins.