There are 5 episodes which bear little relation to the source material they are based on, at least two of these have a totally new plot and characters, these are:
The case of the missing will
Appointment with Death (The murderer has the same name as in the book, but another profession
and motivation. And a supporting criminal, too)
Cards on the Table (One of the suspects are guilty of homicide in the book, but innocent in the
adaption. This character drowns by accident in the book, but ends up with a romance in the adaption)
The Big Four (only loosely based on the book)
The labours of Hercules (loosely based on a short story and expanded)
The other adaptations are generally faithful to Christies works, the short stories sometimes have Japp, Hastings and / or Miss Lemon added with an extra red herring subplot for them to investigate, while the longer novels usually have one or two character subplots conjoined or removed.
On Occasion a minor recurring character, for example Superintendent Spence, will be replaced with a new character due to actor availability.
Because the books were not adapted in published order there are some character plots and themes which are removed from the earlier adaptations and present in the later ones, these included Hastings marriage, his move to the Argentine and Poirots inner battle with mortality, religion, law and morals.