The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
There's also a variation on this where you think it's an origin story about character A, but then the character dies and it's revealed that they were a separate person the whole time, as a way to subvert Saved by Canon.
I'm not seeing it. Might I suggest Bait And Switch Flashback (rather than limiting it to origin story) if it goes to TLP?
^ Sounds good to me.
mwop
There's an abandoned TLP thread called Decoy Origin Story
with plenty of examples.
You think it's an origin story about character A, but it's actually an origin story about character B, who apparently was very similar to character A when they were young.