Occasionally, a writer feels a desire to continue a story s/he had originally planned to stand on its own. To make it more interesting, events that originally meant one thing are reinterpreted to mean something that better suits the sequel's plot. This is retroactive continuity—to those in the know, a retcon.
Note that this is different from deliberate misdirection—in the latter case, the author plans multiple interpretations in advance, before writing the original work, and only the readers are meant to miss the alternate meaning on the first go-round. A retcon is needed when even the author hadn't originally interpreted the events the way the sequel requires.
So, would you do this? Have you done this? How did you go about it? How would you assess your handling of it? How far do you think this technique can be taken?