Properly defining Fi has placed us in a position to ask your question in a manner it could be clearly answered and with reliable results.
Because Fi is simply a tendency to emote in relation to the inner life, it ipso facto lacks logical structure.
Definition of feeling from page 3 of "Gifts Differing"
"A basic difference in judgement arises from the existence of two distinct and sharply contrasting ways of coming to conclusions. One way is by the use of
thinking, that is, by a logical process, aimed at an impersonal finding. The other is by
feeling, that is, by appreciation-equally reasonable in its fashion-bestowing on things a personal, subjective value."
Definition of introversion from page 7 of "Gifts Differing"
"
Introversion, in the sense given to it by Jung in formualting the term and the idea, is one of two complementary orientations to life; its complement is
extraversion. The invtrovert's main interests are in the inner world of concepts and ideas, while the extravert is more involved with the outer world of people and things. Therefore, when circumstances permit, the introvert concentrates perception and judgement upon ideas, while the extravert likes to focus them on the outside environment."
To say that Fi is just emoting is a gross oversimplification of Fi. Fi is assigning personal values to things, people, ideas, etc. Stating that feeling lacks a logical structure is a nice excuse to throw some Latin out there, but since we all know that feeling is not logic the only purpose of making such a statement would be to try to prop thinking up on a higher pedestal than feeling. I do not yield this ground to an argument of definition. I could just as easily argue that feeling is more valid than thinking because that's the way I feel about it.
Fi, unlike Fe that conforms to the rigid structure of the external world of values (therefore does not change easily, it changes in a manner akin to the whims of fashion, in small ways. We notice that significant traditions and values of the community rarely radically change, and hence neither does Fe for this reason.), has no solid foundation. It is founded on mere fleeting feelings. (There is no solid foundation because Fi in itself does not conceptualize feelings, it only processes them, hence there is nothing stopping Fi from going from one temporary passion to another.)
If Fe is a mirror, then Fi is a light. Two mirrors that reflect each other cannot change, but a light can illuminate a mirror and thereby change what it reflects. That's why Fi's must be the change they wish to see in the world.

You are taking a few nuggets and trying to extrapolate them into a whole theory in the span of a single post. Instead, it may be useful to approach the line of reasoning you are attempting instead from the totality of the type descriptions for each type with F and P. I do not think people generally get the idea that the feelings of INFPs are fickle after reading the type description, or by referencing the famous INFPs such as Gandhi, or Joan of Arc. I do not think many would argue that their convictions (based on feeling, not thinking) were "fickle." I know that most of my values based on Fi are most definitely not fickle and have not changed over several decades.
Simply by virtue of the principle that what does not have a solid foundation can easily change, we adduce that Fi established values can easily change.
This conclusion is incorrect. Again, restating definitions that are already a given is not only poor writing style, but does not make your argument any more logical or any more valid. Sorry if I have come off sounding more brash than I normally do, but I have grown tired of posts that seem pointed down barrel of an upturned nose while in and of themselves adding little to our understanding of the human condition.
To answer the OP, my core values are not very changeable; in fact, I value those values that are firm, and use them to grow. The practical implementation of those values on the other hand is malleable. For example, one value I have is equality of opportunity. ("Life, Liberty, and the Pursuit of Happiness.") Does an income tax system create more or less equality of opportunity than a flat tax would? Again, I would point to famous INFPs who have been leader/healer types that have implemented some fairly drastic change that will be noted throughout history such as Joan of Arc. Fi values are not without foundation, and are not "fleeting."
Edit: I wonder how much we are in disagreement due to definitions? I will sleep on this and ponder it some more.