I had the same problem and wrote email to support and got the response below:
The easiest way, assuming your site doesn't have a super large amount of WordPress Multisite blogs, is to manually recreate the multisite blogs, and then use the import/export tools to migrate the posts from your old blogs to your new blogs.
You can find the import/export tools in the WordPress dashboard under the "tools" menu.
You can also manually import the old database.