[CosmoTheFairy]

When I was a kid, I was wild and popular and extroverted. I was a runner and I won national cross-country races. That sort of thing. When I was about five I was obsessed with having imaginary friends until my Mum told me to stop it, because talking to yourself was the first sign of craziness. Lol. I mean, when I was little, if I trusted somebody they'd do something to hurt me. Gradually I got more and more and more introverted until I couldn't run anymore and I sort of sunk into myself. I started high school and I turned into the vulnerable loner girl, who's thing was art and writing. Eventually I grew from that but I was just wondering, why'd I change so much? Could my personality type change again? Has this happened to any of you?

[RetroVortex]

It can change permanently, or it can mask as something else.

I think there were signs I was an INFP, but I masked as a INTP for a long time, then in my teen years I fluxed between the two, before finally accepting myself as an INFP.

[xEmptiness]

Imaginary friends is a very intuitive thing. It is also generally correlated with high IQ.
According to MBTI theory, it is impossible to change your personality type unless you are in a serious accident or something like that.

Only your dominant function is noticeable in childhood, and the other functions only emerge later in life (starting from aux function). That's why you perceive a personality change, but it is rather the result of you not having your other functions yet. It seems like you have a strong distrust for Fe, which is not atypical for Fi doms.

I was extremely extroverted as a child but now I'm the most introverted person I know. I would imagine being a child had more to do with extroversion at that age compared to personality.

[RetroVortex]

I had an imaginary friend once.
His name was Fred.
He wasn't very interesting though, so I destroyed him!

(I'm honestly not even joking on that one.)

[Paradox1987]

I would say yes it can. Your brain pretty much constructs your personality, regardless of the model of personality used. Whilst the brain is pretty plastic in most respects (there are some exceptions, but they're not relevant here). Whilst, subtle changes in preference will strengthen certain neural networks, and dampen, if not wither lesser used/developed networks. Whilst, obviously, damage to or illness of the brain can change your personality dramatically overnight. Whatever happens in your brain will be reflected out to the world eventually :).