I did this once before. In An Embarrassment of Corpses, I make a very convincing case for why Oliver himself should be the murderer, before I switch everything in the final chapter. (I know it's convincing, because I had hate mail from people who were fooled.) And now it can be revealed -- the reason why it's so convincing is that it was, in fact, the original ending of the book. Only in the course of writing it, I came to like Oliver so much as an amateur detective that I wanted to use him again, and I wasn't inclined to turn him into a peripatetic villain, like Patricia Highsmith's Ripley.
(Why am I spoiling the surprise, in case you're among the vast majority of the American population who haven't read the book? Because when the publisher issued the sequel, Murdering Ministers, they attached the phrase "An Oliver Swithin Mystery" to the title, which is kind of a giveaway that Oliver isn't spending the rest of his days in penal servitude. I'm hoping it's like Ron Howard's Apollo 13 -- even though we know that in reality those astronauts got down safely, it doesn't stop us watching the movie with our hearts in our mouth.)
So I had to go back and graft an entirely different set of clues onto the story structure, with alternative motives and a new outcome. I'm rather proud of the fact that while the basis of Embarrassment -- detecting the patterns behind a serial killer's string of bizarre murders -- is a familiar one to mystery readers, both of my resolutions, ersatz and real, were original. I didn't lift any of the solutions used in similarly themed novels I'd read up to that time.*
But I also felt obliged to come up with a reason why Oliver couldn't have committed the murders -- why the reader should have realized that so-convincing false ending had to be false. And that raises a questions of fair play that I haven't thought about before: Is it enough to show why the guilty party had to be the killer? Or is it better if you can also show why each of your red-herring suspects couldn't have done it?
*Agatha Christie's The ABC Murders and And Then There Were None, Philip MacDonald's The List of Adrian Messenger, and Lionel Davidson's brilliant and frustrating The Chelsea Murders. I'd also cite two Vincent Price movies that I had to be careful not to copy: Theater of Blood and the hugely underrated The Abominable Dr. Phibes. The movies The January Man and Seven appeared after Embarrassment was published.