Some feeds subscribers are now back for missing dates.
Good practice to download the CSV of your subscribers as back-up.
To see all of your subscribers: Click on the Analyze tab in FeedBurner. On the top right corner, you should see a drop-down box with the date. Select "All Time" in the box, then go click subscribers on the left menu. When that page loads, make sure it still says "All Time" in the date box. If not select "All Time", then scroll down to "FeedBurner Email Subscriptions". Click that, then click Manage Your Email Subscriber List.
If you then want to move your feed to FeedBlitz - here's how http://www.feedblitz.com/the-feedburner-migration-guide/
I work for FeedBlitz and we'd be happy to assist you with any support you need to migrate your feed and your email subscribers.
Rebecca