Go to Feedburner and click on your blog. You'll see a zero right away, but don't worry. Go to the drop down menu on the date and choose a different date. I chose September 14 and it worked for me.
Then it will pop up a new menu and you can see all of your subscribers again. Click on where it says "See more about your subscribers" underneath the number of subscribers.
Underneath your feed pie, you will see all sorts of things. Go towards the bottom and you will see where it says "FeedBurner Email Subscriptions". Click that. It will drop down a new menu that says "Manage your email subscriber list". Then click that.
It will bring up a new page. Find where it says "Export: CSV" and click on that.
And that's it. It will download a document to your computer, which has your complete subscriber list in it. Then you can just import that right into a new newsletter service that you use.
I hope this helped!