Thanks for the test and the PR Wade, I’ve merged it so the migration plugin will start working for WP 5.4.1 now.
The “advanced controls” section is disabled/hidden if any of the migration plugin’s checks don’t succeed. This is mostly out of an abundance of caution, I think we can just show the button if the only thing that’s wrong is the WP version. This will require a new version of the migration plugin, but it will remove the need to paste special code into the browser console as Wade did.
Only the migration builds will work when switching from WP. We’ll need to document this.