[joconde]

> You don't flip from being extrovert to introvert just because you have a time in your life where you prefer to be alone. Extroverts don't become introverts while grieving the death of a loved one.

These are hypotheses that I wouldn't rely on to give advice. It could discourage someone who is dissatisfied with themselves from trying to change. Who says it's impossible? Your personal experience, or observing people around you, or a study on 1000 people?

[dasil003]

I believe GP is talking about the definition of the word, not commenting on human limitations.

[joconde]

They claim that "You don't flip from being extrovert to introvert just because [...]", so their definition implies that people's ability to switch from one to the other is limited. I don't think that's a reasonable thing to claim without evidence.