Well I think of it kinda like genetics. We walk into this life with a big glob of probabilities. Of those, a certain % are "activated". And that is what we deal with. We wake up in the morning a sorta-vaguely-almost-white-girl because that's genetic. We are better at logic than at tetris, better at recognizing voices than faces, and for those with that experience I expect it's pretty much genetic in base. You might call it destiny from the other side, the causal side.
On the other hand, we have the ability to activate or change our experience based on genetics as well. Certain environmental conditions (and that includes psychology and it especially includes nutritional intake) can trigger changes or deviations in how our genetics play out. We can all eat far too much sugar (carbohydrate) from birth to age 35, but some people will get cancer vs. schizophrenia vs. obesity vs. diabetes vs. early alzheimers vs. other options from the same nutritional cause but different genetic handling of it. We can also put ourselves in highly stressful situations that cause or invoke variance, sometimes not only based on genetics but based on how experience activated our genetics in the past. So for example in a threatening (and especially chronic threatening) situations, many people dissolve, collapse, or otherwise flip out; I actually become far more productive and focused, because in childhood that's the environment I learned to adapt to and rules I learned to work within, and it's partly my genetics and how they cause me to react to some things at play in that. Eg some people both in childhood and adulthood would have a very different response to the same situation than I do, innately.
But just because genetics help 'explain' why factor X is in place in a person does not mean the person cannot change that factor -- depending on what it is. I don't think I can change my skin color (nobody gets whiter except Michael ;-)) (RIP buddy). But of the mass glob of genetics I got, only some are activated because only some have needed to be or have been by my mental focus.
I suspect (here is where we get into mysticism) that archetype work (my favorite hobby, jungian/shamanic stuff) can cause substantial changes inside us and in the reality outside us, a reflection of us so both actually change. I think it is entirely possible that in doing this kind of thing we are working much more with "the larger glob" of our genetics and not merely what has been activated so far. In a video game you might compare this to playing with the full potential of the character rather than just what armor and weapons and powers they have at hand at the moment. To varying degrees it's a little like defrag on the hard drive; some genetic blocks are fixed in place and not changing (I'll never be taller than 5'6 probably given I'm 43!); while the rest can be gathered in fragmented pieces from all over the place inside me, combined cohesively, and re-written to record in a neat and more elegant, better-accessible fashion.
So finally getting to the point of the topic, I suspect that even if certain things seem like unchangeable destiny (eg I'm a little bit Cherokee and that ain't changing), that doesn't mean that we do not as conscious humans have the ability to invoke other aspects of ourselves to change our focus (I can focus on the black-Irish (Northern Ireland) part of myself instead) (or even how I react to, model, or implement that "unchangeable fact" can vary). Most things are not so hard-coded as this example, so there's a ton of flexibility.
From the time I was 5 years old I knew there was a certain thing I was 'born to do'. My entire life was geared to that career. I was a maniac. When I was 16 I had a few hundred songs in a binder and was a guitar/vocals/songwriter freak for years. In my early 20s, during a break I took in Los Angeles to focus on work + hypnosis/NLP studies (to 'deal with' my lousy childhood in myself, not counting the 4.5 hours of commute this required per day), and after two years of sleeping about 4 hours a night, having constant chronic stress, eating once a day at Del Taco at 1am (the only thing open and on my way in my schedule), sleeping instantly after, etc. I'd so trashed my hormonal balance, I gained so much so fast it was almost awe inspiring--and was so fried and focused that in some bizarre way I really hardly noticed until it was too late. And it didn't budge, despite diet/exercise routines that my friends lost brilliantly on. As science research shows, you can hardly make people get fat let alone keep them fat on purpose; it requires hormonal differences which I certainly invoked in my genetics via the "environmental" conditions I imposed on myself in my type-A-overdrive. Anyway so due to these changes I wouldn't leave the house hardly let alone get onstage. Since this wiped out the career that had been my whole life focus, I came about an inch from putting a bullet in my head over it. I finally got over my drama-queen pity (mostly by deciding that joining a nunnery or losing a limb in a car accident could have had an equally drastic effect on my life, and other people aren't babies and deal with those things ok) and got on with my life, and was finally able to just "let go" of my lifetime obsession, and let it be, truly. To just release that 100%.
And I had a total mystical experience after that where I just suddenly saw the whole path of my life. Like a super-fast life-review of sorts I saw how all the events from early childhood on had played into that, and how circumstance and luck, all the older fab musicians who gravitated to me kindly, all the things that inspired me, were part of a 'pattern' that wasn't any accident at all. And I saw the whole path and that it was, in fact, what you might call destiny; I had often fought with myself over was-I-deluded vs. was-I-destined but at that point I saw that it really was part of some higher-self plan and was and would have unfolded in a certain way. But my abrupt decision to leave my karate dojo for a hypnosis school and add massive commute time to my daily schedule, and the massive psychological turbulence unleashed by really intense hypnotic and other work on myself to 'deal with' the fallout from my upbringing, set off a whole cascade of other variables that had not been in place until then. It literally changed my destiny. Not just due to something like body weight but for other reasons too; I no longer had the kind of focus-drive I had before that time, because I didn't have the "demons driving me" (you might say) that I had, yet still wasn't really healthy enough to have that kind of great drive for positive reasons. I could see that it "could, should and would have been" had I stayed on a certain path; but that I had chosen to change that path. And that was perfectly all right.
I once posted of an experience years later where in a vivid dream I went through the events of my entire life, and then was given the option to shift something that would radically change the outcome of who I was, albeit being less pleasant experientially. I chose that, and woke up--to realize that the incredibly detailed vivid memories of my life in the dream were the same up until about age 9 when my mother died, at which point they started to vary more and more until by around age 13 they were a completely different life from then on (I was 30 when that happened).
I consider both of these experiences to be examples of using 'free will' to 'change set destiny'. Obviously unprovable; it's all mystic stuff.
But I still suspect, due to the insight noted previously, that my current destiny is just as much destiny within the framework where I am sitting, as the previous thing was. It's like every decision shifts the monopoly square we are sitting on. I am still the top hat whether I'm on baltic avenue or park place, but my 'destiny based on my genetics and environment and behaviors' is likely to be different depending on which probability(square) I am "within" when I make that same decision.
I really shouldn't be talking about this stuff so early without more caffeine.
