Ok, but how do people change? extrovert to introvert socially.
I believe a number of students in the field are of the opinion that the dominant function and inferior function are developed first and are weakly differentiated at first.
Remember the dominant and inferior always oppose each other in every way (including I/E).
So developmentally, people begin life (more or less) as ambiverts and it can take years/decades to strongly differentiate the dominant from inferior (until this point there is hypothetically little/no differentiation of the auxiliary...). While the first and fourth functions battle for position like this, an introverted person may appear to be more extraverted
and vice versa.
To claim that "introverts' brains are different" is basically the same thing as saying "extraverts' brains are different"; this concept therefore has very little (if anything) to do with what you're trying to describe, other than to assist in theoretically understanding that introversion and extraversion
are fundamentally different, even at a neurological level.
What is supposed to happen, is around the age of 20 we should finally notice clear differentiation of the dominant and inferior functions, with a clear hierarchical organization (i.e. the dominant (
n.) is seen as truly dominant (
adj.)) This is when the rest of the inferior functions* (the rational functions** in irrational types*** and vice versa) grapple for position and we notice a clear emergence of the auxiliary function over the next few years.
Incidentally, this is also the same time-frame for the brain to fully mature into adulthood and
Erikson's personality
identity crisis to be fully resolved.
* "inferior functions" refers to the three out of four cognitive functions that are not dominant (i.e. one of T, S, N, F will be dominant and the other three are seen as inferior). This contrasts with the specific terminology "inferior function" which is almost always referring to the fourth/last function which opposes the dominant most strongly. I have used both terms in context and want to ensure clarity is maintained.
** rational functions are T and F
*** irrational types are N-dominants and S-dominants (INTJ, ISTJ, INFJ, ISFJ, ENTP, ENFP, ESTP, ESFP) // not to be confused with Kiersey's "rational temperament (NT)" as they mean very different things.