It's when someone decides to take most or all of the "onceler" aspects out of their onceler OC, so that they become a regular OC without any ties to the onceler. The change doesn't necessarily have to happen instantly either, like maybe they've been rping and making so many headcanons for their onceler OC that over the months or years their onceler OC barely has any connection to the onceler anymore, not in looks, not in goals, not in backstory, not in personality or interests, nothing, and they look back and realize, hey, it might be fun to officially deoncelerize and go nuts with creativity! But the general etiquette is that a onceler OC is a onceler OC so long as the creator still says so.
But the most important posts from that link are these:
[link 1] The very first deoncelerized onceler askblogs (we called them onceler askblogs back then instead of onceler ocs). It started in our Thneedville High AU. At the beginning it was just people having fun with headcanons for what their oncelers would be like if they weren't oncelers, and gradually it became a whole movement where people actually decided to change their characters permanently.
[link 2] "What is deoncelerization?"
[link 3] When deoncelerization was first proposed, many people were upset. Which is funny since one of the big reasons that deoncelerization was proposed in the first place was because the purists in the fandom were very vocal about HATING the fact that some ppl were letting their onceler askblogs deviate too much from canon movie onceler.
(The fandom by november 2012 was very divided between AU askblog enjoyers and canon movie purists (i personally have always liked both canon and askblogs tho))
And then people became upset thinking that deoncelerization meant the end of the onceler fandom. 😂 No way!!!
There are people from outside of the fandom who have posted onceler fandom analyses claiming that the fandom disappeared overnight at the end of 2012 due to deoncelerization or due to imploding from too many askblogs deviating too far away from canon. This is NOT true. Like sure, maybe some ppl deoncelerized because they didn't want to associate with the fandom anymore, and we respected their decision if so. But from my experience many deoncelerized characters continued to interact with oncelers or other lorax characters, and people in the onceler fandom continued to follow and enjoy the stories of characters who got deoncelerized. The onceler fandom will always be the friends we've made along the way and I mean this so seriously.