I believe all sites should be available in the closure UI.
Then when a site is not actually available for automatic migration, other options should appear.
What appears will depend upon the site and/or why the site is not available for automatic migration.
(This question was added using the mobile site, and so without the benefit of suggested existing questions, or a preview.)
I originally thought this when a code-golf-like question was posted on Stack Overflow itself, and, accessing the site from a mobile, I wanted to quickly suggest that's where it should go. The current UI leaves you with a list of available migration sites and no further possibilities. You have to start over and try to choose the 'correct' option.