Alignment undergoes a big shift from OD&D into the Holmes Basic Set and AD&D. Originally it was a simple three-way system of Law-Neutrality-Chaos, but by Holmes the concepts of Good and Evil have entered into the mix, creating a system with five alignments (Lawful Good, Lawful Evil, Neutral, Chaotic Good and Chaotic Evil). I've tracked these changes below:
Chaotic to Lawful Evil
Chaotic to Chaotic Evil
Chaotic to Neutral Evil
Chaotic to Chaotic Good
Neutral to Chaotic Good
Lawful to Lawful Good
Lawful to Chaotic Good
Note that the above list is probably not precise, with a few accidental omissions here and there. But it gives a pretty good indicator of how alignment changed between OD&D and Holmes (as well as AD&D, which more or less uses the same system for monsters as Holmes).
The alignments previously known as Law and Chaos seem to have mostly split down the lines of good and evil. There are a lot of once-Chaotic monsters that have become Lawful Evil, and quite a few once-Lawful monsters that are now Chaotic Good.
It makes me wonder about the nature of the grand cosmic struggle between Law and Chaos. Though it seems that on a conceptual level those are the two sides vying for dominance, on a practical level it comes down to a struggle between boring old Good and Evil. The good monsters sided with Law, and the evil monsters with Chaos.
What's evident from this is that alignment models a completely different thing in OD&D than it does in later AD&D versions of the game. The Law-Neutrality-Chaos system of OD&D is what I like to call Universal Alignment. These are the only conceptual forces that matter in the grand scheme of reality. Growth vs. entropy, live vs. death, law vs. chaos, order vs. disorder, however you want it to actually play out. The later system models what I call Personal Alignment, which is much more predicated on the internal nature of the individual.
So why the shift? Something happened to fracture the two sides, something that I'm probably going to connect to the rise of various new churches and deities. When the campaign begins, it will be full of churches to Odin and various real world mythologies from Supplement IV. These are powers that are either fading in potency, or not particularly interested in the dealings of mortals. By the time the AD&D era rolls around I will have introduced churches to St. Cuthbert and Pholtus, as those two were the first two such churches introduced in Gary's campaign. And there will also be Demon Lords and Arch-Devils, and the many other staple D&D gods, all more active on the Prime Material Plane than the 'elder gods'. So new gods rise to power and prominence, and the Law vs. Chaos war gets muddied by different ideologies, which makes Personal Alignment (at least on my Oerth) become more important than Universal Alignment.
As for some of the more anomalous results above:
The Displacer Beast above was said in Holmes to be Neutral with Evil tendencies. There's no actual Neutral Evil alignment in these rules, but I threw it in for completeness.
The wereboar is an interesting anomaly, as the only case where Chaos was not equated to Evil. They're Neutral in AD&D, so that's no help. I guess it's just a weird rules thing, but I like having those strange discrepancies. Why did they fight for Chaos, then become Good? Were they enslaved, and then broke free? Or were they utterly loyal to the tenets of Chaos, despite their goodness?