As you probably know, Agatha Christie did not shy away from reusing old plot ideas and devices and put them in a new framework to conjure up new stories in her writings. But we also know that in some cases she simply took an old story and rewrote it - often creating a new novel or novella from a short story.
I thought that in this post I would take a look at some of the most obvious examples of these re-writes, just to keep them handy in one place. I'm not going to list the stories that simply have plot similarities, like Lord Edgware Dies and Peril at End House, or "Triangle at Rhodes" and Evil Under the Sun, or for that matter "The Companion" and A Murder Is Announced. In those cases (and others), while there are similarities in plots or situations or settings, both works stand on their own.
However, there are clear examples when what we're reading is (essentially) the same story. Sometimes they're almost verbatim, sometimes the longer version simply adds more background, characterisation or allows Christie to add in more material but still keeps to the same basic plot, and sometimes Christie would change the stories enough - by changing the culprit or murder method or something similar - to make them fairly standalone from each other.
The listing below is in an approximate order from extremely similar to simply using the same basic structure. Some spoilers are unavoidable.
We'll begin with "The Regatta Mystery". This short story exists in two versions, one featuring Parker Pyne and one featuring Hercule Poirot. The former is apparently the original one, according to research from John Curran, though it was long thought that it was the other way around. These two short stories are almost completely alike, simply changing some names and some of the characterisation necessary for Pyne and Poirot respectively. Reading one will completely spoil the other.
Next, we have "The Submarine Plans" and "The Incredible Theft". The original short story and the later novella are also very similar. Being a novella, the later version obviously includes more material and allows Christie to expand on characterisation, settings and other descriptions, but on the whole they are very much the same story.
One recently discovered story is "Hercule Poirot and the Greenshore Folly", never published during Christie's lifetime and an early version of the novel Dead Man's Folly. Here, the main plot and the setting is very similar. The novel is simply a more fleshed-out version of the novella.
There weren't too many changes between "Christmas Adventure" and "The Adventure of the Christmas Pudding" either. The longer novella simply allows the reader to experience the Christmas setting more fully.
While the main plot of "The Mystery of the Baghdad Chest" and "The Mystery of the Spanish Chest" remains the same, there are some obvious differences. The main one is using Hastings as a character (and narrator) in the earlier short story while the latter features Miss Lemon instead. Having read one will mostly spoil the other story.
Then we have "The Plymouth Express" and The Mystery of the Blue Train. Both
stories have the same plot and the murder mystery has the same
solution. However, there are a couple of fairly significant differences between
the short story and the novel, mainly in the addition of several
characters in the latter, and while the murder method is the same,
additional culprits are featured in the later novel.
"The Market Basing Mystery" and "Murder in the Mews" differ somewhat in choice of victim, setting and characters, while the main plot is still the same in both stories. Again, the longer novella gives Christie the opportunity to expand the bare bones structure of the short(er) story.
"The Case of the Caretaker's Wife", "The Case of the Caretaker" and Endless Night is another case where Christie changed the detective for the final product (or rather removed the detective entirely), though the basic plot is exactly the same in all three variants. This is the only case where we have three different versions of the same story, though the first variant was only recently found, never having been published in the author's lifetime. The first two versions both feature Miss Marple and use very similar plotting, though the first variant uses a narrative technique where Miss Marple is front and centre of the action, while the second simply allows her to read an account of the events and then figure out a solution to them. Meanwhile, the novel is written as a first person account from one of the main people involved in the mystery, styled as a psychological thriller rather than the more regular fair play mysteries that we find in the first two variants. While the narrative style of the novel is very different from the earlier two versions, the reader will be spoiled for the solution by reading either of them.
Yet another case where a different detective is featured in the rewrite can be found with "Yellow Iris" vs. Sparkling Cyanide. The earlier short
story features Poirot as the problem solver, while Colonel Race is the
recurring character allowed to participate in the novel's
investigations. The main situation is the same, as is the murder device.
However, as usual Christie uses the longer version to flesh out
everything, allowing the reader to get closer to the suspects. More
importantly, she also changes the murderer which means that the reader
can still read both stories and get surprised.
Finally, we have "The Incident of the Dog's Ball" and Dumb Witness. Both stories have the same setup and use the same device for the murder. But the latter, being a novel, is much more fleshed out than the earlier short story (another one never published during Christie's lifetime), bringing in more characters and a different murderer. Again, you can read both without being spoiled.