Planescape certainly has its share of fallen Celestials, rogue Modrons and the like. For the most part these are individuals who have broken out of the usual mold for their race to pursue a life under a different core philosophy or ideal from their kindred and plane of origin. 'Fallen' celestials (e.g. Baalzebul, Adimarchus and Xenghara) tend to have much more evil-looking physical forms than they did prior to their fall. Rogue modrons, which have become somewhat less Lawful, will automatically reform to resemble quadrones. Naturally, many other such examples exist of permanent shifts in the physical and intrinsic nature of aligned beings whose alignments have shifted.
Evil creatures can also have their standard alignment subtypes replaced with the 'Good' subtype through a year's magical isolation under the Sanctify the Wicked spell (Exalted Deeds). Presumably similar spells could exist for different alignment shifts as well. A Sanctified fiend has a new alignment and a new 'Good' subtype, though its physical appearance does not change.
I have always seen alignment subtypes as representing the default hard-wiring of certain creatures, i.e a yugoloth has the evil subtype because the creature is a physical expression of the ideal of neutral evil and is bound to the Lower Planes. The subtype helps the Plane to defend itself by driving its inhabitants to act like an immune system against invaders. But how constraining are alignment subtypes upon the individual alignment of the subtyped creature? Alignment in D&D is represented as a series of discrete categories. Practically though, one outsider may be much more neutral than another of the same kind while still retaining the same alignment. Accordingly, alignment is better viewed as a continuum along two different axes with categorical boundaries.
As an example, could a Tanar'ari adopt an alignment other than Chaotic Evil even though it has the Chaos and Evil subtypes? If it stopped acting chaotically for an extended period, would that eventually cause it to lose the Chaos subtype and become a fiend of neutral evil? How long would this take and what would be required? Would such a rogue demon still be able to access its spell-like abilities, or would they stop working due to a weakened connection to the Abyss?
I found it a bit difficult DMing a group including a Succubus for this reason. The player tended more towards neutral evil in how he played his character. I could never quite decide the extent to which the demon's Chaos subtype should drive it to behave in certain ways, and what would happen to it when it stopped doing so. Any thoughts or ideas on game mechanisms to represent what a subtype-aligned creature can and cannot do and what happens within the grey areas are much appreciated.
I've heard fiends, modrons, and other such races referred to as "exemplars" before. There's a reason for it: they are meant to be exemplary representatives of their alignments.
An exemplar being that is not of its 'proper' alignment (CE for tanar'ri, LN for modrons, etc.) is fallen/risen/rogue/whatever you call it. So yes, you can be an NE succubus, but there is a price to pay for it. One of which would be the reactions of other tanar'ri!
I'm not sure whether an NE succubus would lose some of its spell-like abilities, though I would guess not.
Personally, I wouldn't normally be into playing an exemplar being for just this reason. A non-rogue exemplar being played true to form is pretty constrained in its behavior (alignment) and, because of that, lacking a certain depth of which mortals are capable. That's not to say they can't impersonate other alignments (just look at A'kin), but ultimately an exemplar is very much a representative of its alignment.
BoGr Guide to Missile Combat:
1) Equip a bow or crossbow.
2) Roll a natural 1 on d20.
3) ?????
4) Profit!