Has Anyone Here Consciously Changed Their Passions?

My answer to your first question was, I don't know, but in the interest of comprehensiveness I mentioned a minor exception, which you asked about, so here is more info on the minor exception.

what does encouraging/discouraging sources of intrinsic motivation look like for you?

It looks like making a habit of patiently watching for the desired change. My software environment is organized enough that I can usually arrange for my future self to re-read a written reminder. So, (years ago) I wrote a reminder to watch for any instance where I think my behavior and my decisions are being motivated by interpersonal altruism or I experience pleasure or satisfaction from having achieve an altruistic interpersonal outcome. Note that this did not result in incontrovertible evidence of a significant increase in frequency of altruistic behavior. But I certainly stopped my addiction to the flow motivator (over the course of many years, except I relapse when I'm under stress, but these years it takes a lot of stress) and am pretty sure that the patient watching strategy helped a lot there. (And "just watching" helped me make other kinds of internal mental changes.)

My mind seems to works such that if the only conscious effort I make to effect some internal change is to get into the habit of watching or checking to see if the change has already occurred, my subconscious sometimes seems to figure out a way to effect the change if I watch long enough (months or years).

There are much faster and more potent ways to increase motivation and drive for most people: avoiding all exposure to light between 23:00 and 05:00 every night; getting as much very bright light as possible during the first 3 hours of wakefulness; making sure to get enough tyrosine (a dopamine precursor); deliberate cold exposure; avoiding spending too much of the day in perfectly-safe pleasurable activities; doing enough things you find aversive or outright painful; doing enough risky things. But you didn't ask about that, I don't think: "passion" almost always refers to intrinsic motivation (i.e., basically something that feels good or that a person wants to do for it own sake rather than as part of a plan to achieve some other outcome), whereas the motivation for most of what I (and probably you) do is extrinsic. E.g., I take the time to cook a healthy meal not because I enjoy cooking but rather because I anticipate that eating well will pay dividends mostly in future years in helping me achieve outcomes that I haven't even decided to pursue yet. I took stuff like that to be outside the scope of your question.

I can certainly see the appeal of social pressure/the potential reward of better social standing for sticking a long-term goal through.

That is not really what motivates me. It's that when I work on something alone, I feel lonely. If I can talk about it to other people, I don't. Also, I find it easier to focus on things when I can talk about them.

I feel that I am expected to bail by default

Expected... by yourself, or by others? For example, I find talking to some people helpful, but talking to some people harmful.

One way some people can disappoint me as talking partners is when they immediately start predicting that I will fail. "You always talk about doing things, but you never finish any of them. This time it is certainly not going to be any different." This hurts in two ways: on one hand, because it is uncomfortably close to truth; let's say that I finish maybe 1 out of 20 things that I start doing. On the other hand, because it is literally false; I actually do finish 1 out of 20 things that I start doing, and I always hope that this is going to be the one, or that the ratio will start improving.

A glass 5% full is still not the same as empty! I may feel on most of days like a loser, but sometimes I look back and see an accumulated record of successes. If I told someone only about the successes, and not a word about the failures, they might actually consider me impressive. And when we look from outside at others, isn't this kind of filtered view that we usually see? Both of these perspectives can be true simultaneously. I had to learn to stop talking to people who are predictably negative. (Which is different from betting. Yes, when I start a project, I would rationally bet that this project will probably fail. But the point is that some things are worth trying even if the probability of success is smaller than 50%.)

Another way of disappointing me is when the other person tries to takes ownership of my project. When they start giving unsolicited advice, and then get defensive when I don't accept it, often because they completely misunderstand my motivation for the project (am I doing this for myself, or for others? do I want to achieve a specific goal, or to practice a specific skill? which parts of the project are the ones that I am looking forwards to do, and which are the annoying parts that I simply need to overcome?).

What I need instead is someone who would listen, be gently encouraging, maybe give an idea or two, but be perfectly okay with me saying no. Basically, something like a (Rogerian) psychologist. Someone who would remind me of what I said yesterday or a week ago, but would not express disapproval if I failed to do that or changed my mind. Shortly, positive motivation, not negative. Celebration of success (and partial progress), rather than fear of failure. Removing the pressure, rather than increasing it.