Apt-secure and upgrading to bullseye

[Julian Gilbey <jdg@debian.org>]

Hooray, buster's released!  Congrats to all!

So I tried upgrading my machine to bullseye today, and
aptitude/apt-get update don't like this, giving me errors such as:

E: Repository 'http://ftp.uk.debian.org/debian testing InRelease' changed its 'Codename' value from 'buster' to 'bullseye'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.

So I read apt-secure(8), which gives no indication of how to "accept
this explicitly", and neither do any of the linked wiki pages.

Any suggestions?

(And obviously something needs changing so that people aren't stung
when bullseye is released.)

Best wishes,

   Julian