Thankfully, WordPress.com allows you to migrate your subscribers with one condition. You must use their Jetpack plugin, which adds the same subscriber functionality that you had at WordPress.com.
Once you install and activate the Jetpack plugin, you will need to contact the WordPress.com team and ask them to migrate the subscribers for you. We hope that in the later versions of Jetpack, this will become easier, and users will be able to do it themselves.