Here is the definition of “retcon” from Oxford Dictionaries:
I can find other, operationally identical definitions for you.
Not my point, but a cheap imitation thereof. The scar would have come-up sooner in this story exactly because association of the change in her behavior with high school came-up sooner.
So far, this story hasn't ever been a romcom; it's been a story about a fragile person on the edge of shattering. Rom? Presumably. Com? Not in the least.
Sorry, but I still don't understand what you want to say....
By the definition you quoted and your first message, that I quote:
It's a retcon pivot. The original set-up was unsustainable; at some point nearly all readers would hate the MMC.
It would mean, just by reading this, that in the first few chapters, it was stated that her change was caused by him.
But before this scar reveal, we were only told that she changed, not why. Which means that knowing now that the attack in middle school and the consequent scar are the reason why she's now gloomy, is in no way a retcon, since, as per the definition you quoted, we have NO "interpretation on previously described events".
This scar is, in much simplicity, just new information.
And it's not even a retcon if you want to push the "it happened in middle school, not in high school" card, because without any more info, at least atm, it could be possible that she left school to recover and MMC only met her again in hs.
This really made me check all the previous chapters and even the
twitter stories version , just to find out there's nothing about this