I've thought about this at times, when it comes to my own personality. In my early 20's, I would test as INTJ and even recognized much of myself in the INTJ descriptions - at that time. Now? Not at all. (however, there was also a lot about the theory that I did not understand back then, and this included F stereotypes, and my taking the test thinking that there was no way I could be an F; but, as a child, there was almost zilch NT temperament to my personality - so perhaps I was just in my Ti phase in my early 20's).
Because much of my life to date has been internal, it's highly possible I've done so much thinking/reflecting/philosophizing/etc that I've consciously switched some of my functions and rejected others, based on life experiences that have come my way. I've thought about this. I do think that a lot is possible when it comes to re-ordering/rewiring your internal world and perceptions, if you so desire it.
And to me, if over time someone does 'balance out' many of their functions, and their personality becomes less extreme, then can't they, for all intents and purposes, 'be' a different personality at that point?? I mean, they're certainly different from where they started. I guess it depends on how one wants to use mbti theory, or how one views it. If one is using the theory in an attempt to predict behavior and look for overall patterns, then if a person has evolved over time, far enough from where they started, so that both others as well as themselves view themselves in a new light...then in effect could they 'be' a new personality? (in terms of the 16 types - obviously they're still the same living being) Maybe. I mean, say someone's starting point is INTJ. By the time they're 60, they may have shed so many of the typical INTJ traits, and brought in many other traits to complement their 'baseline', that in effect, are they still an INTJ? What if when they're 60, they read the INTJ description and it's meaningless to them because they don't even behave like that anymore, or have broadened their functions so much that the description (and even theory?) is worthless?
Oh, but I know most of the other arguments about this, that one is always the same personality, that never changes, under stress the 'true'/more instinctual traits will come out..etc...people tend to balance and mature over time, and an INTJ at 60 is going to still have a distinctively different flavor than an ESFJ at 60...which is true. Sorry, I think I'm wasting everyones' time here.
I'm just musing.

There are definitely many other ways of looking at it. These are just some of the weird thoughts I have. ;-) Basically, I view the human personality as flowing and evolving over time. I guess too it depends on how one as an individual views themselves. Many might not even desire to 'grow' or balance themselves out, in which case they'll stay pretty constant. Some might view building up ones' strengths as the most effective method of reaching their potential, which would also mean they wouldn't suddenly 'morph' into another type; others might view working on lesser traits as the more effective goal, and that's where the personality type might become murkier.
I go back and forth quite a bit on the usefulness and validity of many of these personality theories, but it's definitely interesting stuff.
P.S. I completely understand if no one can follow what I am saying!