I’m not a psychologist/psychiatrist, but isn’t personality not very mutable past the age of around 7? At least without psychedelics or brain damage or something?
I’ve read that this is a common myth and that personality continues to change throughout our lives (1, 2). According to some psychologists, this could be related to the end of history illusion.
The book Personality Isn’t Permanent talks a lot about personality myths and ways to change one’s personality. In the section on how to change one’s personality, he suggests (albeit doesn’t cite studies) that the reason personality changes less in adulthood than childhood may be because our environment becomes more constant and environment can shape personality.
This seems to be almost exactly what you are suggesting. At the end of the article he seems to offer an open invitation to receive their vaccine. While there is no mention of their vaccine’s efficacy in the article, the article was written in late July so the team may have more updates by now if you reach out to them directly.