The fact that people were registering .ml domains for projects like this is mindboggling. There are many TLDs to pick from without infringing on the terms of use of a country-specific one.
Well, this is just weird. When I was migrating from Reddit to this fediverse world I chose .ml and thought it was short for "machine learning" which seemed as a cool domain for me at the time.
On the technical topic of renaming a domain of a Lemmy server... I think it is worth experimenting with the code. At minimum, I think it should be an option to try and keep the same login/passwords for users from the old install of Lemmy. But even that could prove tricky if a particular domain changed underllying ownership more than once - and user@domain became rewritten by an entirely different person. I guess in the real-world people do often get mail for previous residence of a house.
My biggest concern is legality because Lemmy claims to support privacy. I honestly think it's a bad idea to claim privacy because you run into so many problems. If the user never knows that their lemmy instance changed names and can't find it again, etc. Especially on technical topics, 15+ years of having Reddit keep messages from deleted user accounts offered a lot of great search engine hits. With Lemmy, a person moving to a different instance and deleting their account, so much content is going to get black-hole in favor of 50 instances having copies of a meme post or trivial website link - and solid original content (often in comment discussions) gets removed.