It’s more complicated than that, and it will completely break federation for all instances that have already federated with yours.
https://join-lemmy.org/docs/administration/backup_and_restore.html#changing-your-domain-name

That’s what https://lemmit.online is.
I think it is a great idea to have a bot mirroring Reddit’s content onto a lemmy instance. But I agree, we don’t need those bots mucking up actual lemmy communities.