Today, I finally transitioned from my 1024 bit RSA/DSA key to a new 4096 bit RSA key. Following http://www.apache.org/dev/key-transition.html the procedure was quite easy.
The old key will expire on 2012-12-31. Having two valid keys in the Gnupg keyring is not the best way to go for the automatic key selection in Thunderbird. It insists on using the old key, therefore a manual selection is required until the old key can safely be removed.