Okay, so here's where it got weird...
Not much time has passed between the end of the trilogy and the beginning of the fourth Flambards book, though it was published twelve years after the trilogy was completed and two years after the television series. In fact, this book came out about the time that I discovered the world of Flambards, since the series was being shown on American public television in about 1981 and after I watched it I also found that I could purchase a one-volume version of all three books covered in the television series. (It was at Walden Books, with a photo on the cover of the actors playing Will and Christina, in a still shot from an episode that occurred during the timeline of the second book.) The single-volume trilogy had obviously been published to benefit from the popularity of the PBS broadcast of Flambards, but given that it was 1981 and the author had just decided that she had more to say about these characters, it was a somewhat glaring omission to not include any information about the sequel to the trilogy.
Unless--the author's fourth book was some sort of reaction to the series. One has to wonder. She never felt moved to do this until two years after it was aired on television in the UK. But more on my speculation about that later.
Here is the way the ships shifted and morphed in the fourth book:
Christina/Dick - Times are bumpy for these two, complicated by the war, raising Dick's nephew as their adopted son and a miscarriage that sends both of them into a deep blue funk, which is only to be expected. Partly because of this and partly because Dick has never really felt comfortable interacting with people he was raised to think of as his "betters" (Mark, the other landowners in the district), he finds himself spending more time with a local girl he knew growing up, someone with whom he feels comfortable and easy, unlike his wife's milieu. Her "people" are the wealthy folks who looked down on him when he was a groom and who consider him an upstart now that he is married to a wealthy landowner and running his own farm. I could well imagine K.M. Peyton (though I've never read an interview with her) saying that these two need some "couples counseling" if she were to comment on their marriage today.
Dorothy/Mark - Dorothy and Mark are, of necessity, having a long-distance marriage while he continues to fight in the army and she continues to nurse. However, when Mark is badly wounded and sent back to England to recuperate (which will last well beyond the end of the war), he is at "home" again, while she is still at war. There is ultimately nothing for it but returning to Flambards to recover, though he has technically sold it to Christina and has no claims to the property anymore. She is still family, and his hospital bed is needed for another wounded soldier, so "home" he goes. Naturally, this doesn't go over well with Dick, with whom Mark has never got on, and Mark likes even less living in a home technically owned by Dick that was once his, relying on Dick's largesse and generosity. Dorothy and Mark eventually divorce, but that is more due to their long-term separation and not having had much of a courtship to begin with, plus an affair of Dorothy's, which she means to turn into a second marriage. But first we need to address--
Christina/Mark - It falls to Christina to nurse Mark through a very bad recovery, and while neither she nor Mark have the temperament for this not to be rocky, the experience does draw them closer. What binds them even more closely together, though, are his relationship with Tizzy and his eventual collaboration with some army chums interested in cars, a new pursuit of his. Christina isn't as worried about his injuring himself driving as she was about Will being hurt flying, and Mark also gets on fairly well with all of the neighbors, who were accustomed to hunting with him while he was growing up. The only obstacle to their being together, after Christina and Dick divorce, and Mark and Dorothy divorce, is that there needs to be an act of Parliament to make it permissible for a man to marry his widowed sister-in-law (something to do with the Bible--I don't think we ever had a law against this in the US). But once it's passed, which is deemed very necessary to the country, due to the numerous fatalities from the war, they can finally be together.
Why the ship change?
I have no idea why Peyton felt the need to write this book, especially as having Christina and Mark end up together and having her marriage to Dick fail specifically because they are too different, from different social strata, while she and Mark are deemed more compatible because of class similarities, contradicts the somewhat radical messages about class stratification that Peyton was putting forward earlier in the series. But we end up with Dick and the girl he knows from the village, two "peasants" together, as Mark would put it, and Christina and Mark together, a match his father originally wanted mainly because of her inheritance.
Did Peyton become disenchanted with Christina/Dick because of something that happened in her personal life? Did she see the television series, think the actors playing Mark and Christina had more chemistry than Dick and Christina or Dorothy and Mark and decide to continue the story with rearranged couples?
I have no idea. And I have to say, I don't really like Flambards Divided. The complete turn-around from her earlier support for challenging the status quo and questioning class divisions disappears; it is thematically very far away from the trilogy. I suppose it's also possible that there was a general clamor for her to write more in this universe, and this was her revenge: something that would guarantee that no one would want her to write about these characters EVER AGAIN. (Okay, maybe that would depend on whether you ship Christina/Mark. But it COULD be revenge for forcing her to write another book; think of what JJ Abrams did at the end of the fourth season of Felicity when he was forced to keep making episodes after he thought he was done.)
So, I don't know what will come out in the full text of JK Rowling's interview tomorrow. But unless she proposes writing an eighth book in which Ginny and Ron both cheat on their spouses and Harry and Hermione hook up (yeah, I know--you've probably read the summary for that fic), the seven books of canon are inviolate. What happened happened. It's over and done.
Trust me. It could be worse. Flambards Divided exists. It's a real thing. But I don't expect JK Rowling to go down that road. I firmly believe that she's done writing canon with Harry's generation, and if she does next-gen stuff I'd be very surprised if she changed the ships for Harry's generation. She could very well say in this interview that she now thinks Harry and Hermione would have worked out better than Ron and Hermione. I don't know yet but I don't think it matters. That's not what she wrote, after all. (I have to join another commenter I've seen online wondering whether her childhood friend upon whom Ron was based has somehow done something to upset her greatly. If he has, though, it's a very weird way to tell him, "I'm upset with you," through doing this interview.)
Now, because we could all use a laugh,
this was how Stephen Colbert* covered this story earlier in the week.
* Remember: Colbert is playing a character. He really believes the opposite of whatever his character says.