I've recently upgraded a Debian testing machine that has been lying in
a closet for a few months.  As expected, APT warned me about
unverifiable packages, and I spent half an hour googling around in order
to find out how to import the new keys.

Since most people don't upgrade abandoned machines very often, this is
something one tends to forget, and needs to look for every time it
happens.

APT should say something like:

  "Perhaps you should upgrde the debian-archive-keyring package?"

                                        Juliusz