Today is the deadline to have all your FeedBurner accounts moved over to your Google account.
Although I've heard various opinions on whether the old Feedburner subscriptions will continue to work – with some bloggers reporting that subscriptions should work indefinitely and others already reporting glitches with the old addresses – I'd trust the Feedburner to Google FAQ which reads:
“It is our intent to keep backward compatibility with your feed address as long as this service exists.”
I updated my accounts last week and haven't yet changed all the feed addresses, but the old one's are redirecting just fine.
The real issue is in updating your accounts. According to the FAQ, if you don't update your account:
“…you will no longer be able to access your account at http://www.feedburner.com, and your feeds will either begin returning a “404 Not Found” response or a “301 Moved” response back to your source feed.”
Learn more on the Feedburner to Google FAQ