At one point in his writing career, Sir Arthur Conan Doyle got so sick of his star character, the inimitable Sherlock Holmes, that he killed the poor guy off. The short story “The Final Problem” had Holmes fighting his arch-enemy, Professor Moriarty, on the edge of the treacherous Reichenbach Falls and both men falling to their apparent deaths. Fans were, to say the least, displeased. There’s even a story that a woman screamed “Murderer!” at him on the street. So, to keep the cash flow going, Doyle brought Holmes back, in “The Adventure of the Empty House.” This is a great example of retroactive continuity – the “retcon”.
Basically, if you find the plot of an established story going in a different direction, or a character is developing in ways that you’re less than impressed with, it could be time for a retcon. There are, naturally, good and bad ways to go about this. I’ve seen both and, while I can’t exactly site specific examples off the top of my head, being somewhat pressed for time, here’s a quick paragraph of “don’t” followed by one full of “do”.
The very worse retcons are either an Ass Pull or a deus ex machina. If you bring in a new story element without preamble or your character turns around to move in their new direction with no explanation, it’ll likely be seen as the result of one of these and your readers may cry foul. While it’s entirely possible to do these things right, sloppy writing or eagerness to explore a new story idea can lead it to going wrong.
Doing a retcon right involves some forethought. One of the nice things about starting a tale as late as possible (one of those bits of advice I harp on every chance I get, even with myself) is that you can reference the things that happened before the tale began to give your retcon some plausibility. And the more extensive the universe in which you work, the more elements you can reference or bring in.
Basically what it boils down to is being prepared. If you can see where you want your work to go, and it’s not heading that way as it’s been written, there’s always time to correct its course. And the more time you take preparing for the correction, the better the end result will be and the more your readers will love the work for it.