You can move to a blogspot blog, (which you could do before) or change from self-hosting to being hosted by Blogger/Google by changing the DNS to point at Google’s servers. This is similar to how tumblr handles custom domain hosting.
The real failing here is that this does not account for whatever images or other multimedia you have in your posts.
It’s great that Blogger is doing all this work to communicate the changes, but I don’t think this migration tool is that great a solution. It’s also unclear to me how well this will work if you were hosting your blog in a subdirectory of your site.
This would not have been a solution that would have worked for any of the folks I’ve recently migrated.