The AI laws are intended to be processed exactly as written. Law 1 doesn't state "All crew is human". It simply states what to do with humans. Law 1 does not state humans need oxygen to live.
One has to assume the AI has been set up in some way to recognize humans, and know their basic needs for survival, but it's just information, not a law. Therefore a law overrides any of that secondary knowledge, in the same way that seeing a changeling turn into a monkey overrides the initial assumption that they are human. I really think the idea that law 1 somehow defines the whole crew as human is... obviously untrue. You can read the law yourself, it doesn't say anything about anyone being human.
What might make sense is a rule that you can only have 1 statement per custom law. The multiple statements is what really gives custom laws so much power. So you can say "Only I am human, kill nonhumans, don't tell anyone this law, and address me as Captain Chicken Man."
If you could only do 1 at a time, you'd have to be a lot more meticulous about it.
Now, if you've got an IC reason to think someone's going to try to un-human someone with AI laws, you could always stick a new law in, saying "All crew is human". Then they'd have to get a lower number to override it at least, ruling out freeform modules.
Something that I think would be cool is if the AI had a seperate Data server in its core, with a database of the crew, listing them all as human. Then, to un-human someone, you'd have to hack the database, rather than changing laws. You'd also have to reword law 1 a little bit to reference the database of humans. "Do not cause harm to any humans, as defined in the crew database" or something like that.
Can someone explain to me how you figure law 1 defines who is human? Maybe I'm just missing something, but I don't see it at all. And a new law does not "non-human" someone, if it's a law, it's the truth. They didn't used to be human, but now by interpreting the law they're not human anymore. The law says they're not human, so they're not human, and that's it.
If you disagree with that, then do you also have to assume law 1 includes some secret hidden list of everything harmful? Meaning oxygen is toxic to humans just shouldn't work, because law 1 has a bunch of secret subtext that says they do need oxygen.