It seems clear from the metaethics posts is that if a powerful alien race comes along and converts humanity into paperclip-maximizers, such that making many paperclips comes to be right_human
No one can change right_human, it's a specific utility function. You can change the utility function that humans implement, but you can't change right_human. That would be like changing e^x or 2 to something else. In other words, you're right about what the metaethics posts say, and that's what I'm saying too.
edit: or what jimrandomh said (I didn't see his comment before I posted mine)
What if we use 'human' as a rigid designator for unmodified-human. Then in case aliens convert people into paperclip-maximizers, they're no longer human, hence human_right no longer applies to them, but itself remains unchanged.
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.