Click on your name in this thread to take you to the screen where you can do it. The one you previously uploaded is for the public profile page. The tab will be at the top right.
The database might be a bitch to transfer.
One reason I don't use this site's album or such features but link from my own server or Flickr; less stuff to get lost or be forced to relink/reload.
A missing avatar should be a simple matter to reload. As long as we don't lose our post count, I'm OK.