https://daringfireball.net/linked/2025/02/11/about-migrating...
I submitted a blog post on how to merge manually about 4 years ago:
https://news.ycombinator.com/item?id=27797535
https://brianstucki.com/how-to-manually-merge-two-apple-ids-...
I'm sure glad I procrastinated on that.