There are four classes of (sapient) being: outsiders, "guardians," mortals, and aberrations (creatures from "outside" ordinary reality). There are also non-sapient animal beings, but those are kind of outside the scope of "exhibits moral behavior," and thus irrelevant for discussing alignment.
Outsiders are spirits who can manifest physical bodies, whose fundamental nature is at least a little alien and eldritch. It is possible for them to change alignment, but it is a dramatically more difficult and major change than it would be for humans/mortals and other "guardians." An outsider that truly changes alignment (rather than being gunked up by mind control/curse/etc. or masquerading) literally becomes a different kind of outsider as a result. To be a succubus is to be evil (indeed, as a demon, they fought in the infinite-yet-instant War in Heaven in order to be evil), so if a succubus becomes non-evil somehow, it necessarily ceases to be a succubus.* Such is the case for all outsiders; if they change their alignment, they inherently change what they are, because (to use Aristotelian terms) their telos (purpose/end) and their aitia (causality/responsibility) are inherently linked.
"Guardians" are a nebulous category not fully defined yet, but they seem to be a midpoint between outsiders and mortals. They can be immortal, often have much greater power than mortals can usually achieve, and usually seem to believe they have a mission or purpose given to them (hence the name, though the quotes are used because the party doesn't fully understand the issue yet, and neither do I.) A "guardian" can change their alignment freely like a mortal, but it seems to have an impact on them, like an outsider. Perhaps it is a matter of making an overt choice, perhaps it can be a slow creeping change, hard to say. The party has only met a relatively small number of them, and doesn't actually know that all of them are "guardians." Gold dragons are one example, and (apparently) a black dragon is a fallen draconic "guardian."
Mortals are ordinary folks, but this includes some people we might not consider strictly ordinary, such as genasi, tieflings, and aasimar. The party bard is a tiefling on both sides (one devilish, one demonic), but has gained a great deal of power--he may be moving in new directions or perhaps even making a transition into "guardian" status. Mortals can change their alignment however they like without any physical changes whatsoever; it is part of a mortal's nature that they can change in this way. Mortals, unlike celestials, do not stay on their plane of origin when they die. Their spiritual energy eventually returns to the land, but the person is gone--some (the Safiqi priests) believe the actual self-hood departs the circles of the world for Jannah ("True Heaven," as opposed to Al-Jana, "the heavens" where the stars etc. are), while others (mostly the Kahina, and some atheistic Waziri) just think the self dissipates or acts as a karmic ripple on the surface of existence.
Aberrations--what 4e would call "Far Realm" denizens--are truly alien and usually hostile to life. They generally do not live in the way ordinary things live, but they can die under mundane circumstances unlike outsiders. Their existence is enigmatic and they don't, as a rule, form social structures or cultures the way mortals do. Best policy is usually to avoid them, as many are frightfully intelligent and very good at manipulating people into helping them...but their intelligence often brings equal pride, so exploiting their hubris is possible. Few truck with such entities regardless simply because of the danger.
TL;DR: Hard-coded alignment only applies to outsiders and aberrant entities, the former because it's part of what it means to be an outsider of that type, the latter because they're spooky, scary eldritch beings. Soft-coded alignment applies to "guardians," who seem to be a midway point between outsiders and mortals. Alignment is not coded at all for mortals, because "is a mortal" directly implies "does not have innate alignment." (Technically there's also non-sapient animals, but they can't have alignment in the first place, so they're kind of irrelevant.)
*This actually happened in-game. A succubus (ancestor of the party's tiefling bard) changed her true name without even realizing it, only finding out when she attempted to invoke her old true name and it didn't work. That old name refers to a different being who no longer exists, because she's not truly a succubus anymore. She's become something else--and by giving up the last of her succubus powers (to said bard), she's truly mortal now, other than the inexplicable retention of her supernaturally beautiful singing voice. (She takes on faith that this is a sign from the One that her penance has been accepted.)