As I understand the story, Christie made it so several characters could be the murderer, then decided at the end which of them it would be. So, since all were a possibility, she didn't really have to do a whole lot of rewriting. Honestly I haven't read that many of her books, and can't say if the story is true, or just true in some cases, or not true at all. The way I see it, the best mysteries are the ones in which the reader can't see how any of the suspects fits, but when the criminal is revealed they slap themselves on the forehead. "Oh--it's so obvious now!"

But Conan-Doyle didn't work that way. It wasn't uncommon in the Holmes stories for the reader to have absolutely no clue what was going on, until Sherlock revealed details the reader hadn't been told about. Personally, I prefer to feed my readers enough information that they at least have a chance to guess who the bad guy is.

The important thing is to offer enough suspects, and enough twists, that you don't make it too easy for mystery lovers!

Murder She Wrote is still popular over here, and I watch it from time to time. One of the cable stations runs it most of the night--and even though it looks older, the stories still hold up.