Thought I'd mention this, just upgraded a second blog from 2.3 to 2.4. After migrating the website.handle field contains the admin's handle and not the website's handle; and the admin user appears twice in the blog's member list (because in 2.3 they were both admin and editor roles and now it looks like the duplication isn't needed).
Easy stuff to fix in the db, but thought I'd mention it. Hen
