I started a snippet to explain how I’m doing: Semi-automatic steps to migrate merge-requests between projects in GitLab ($2130375) · Snippets · Snippets · GitLab
In particular, the public API documentation doesn’t explain about how to create diff notes, for example. But I just debug what the GitLab javascript was doing when I start threads and create a comment over a commit line in the changes tab.
It’s a very simple work yet but could help someone else. It solves my situation. It was difficult understand why some threads are not mapped as discussion, I dropped some comments in the code.