"apt-get update" says:

E: Repository 'https://debian.ethz.ch/debian experimental InRelease' changed its 'Codename' value from 'experimental' to 'rc-buggy'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.

But apt-secure(8) does not explain how to actually accept it:

  INFORMATION CHANGES
       A Release file contains beside the checksums for the files in the
       repository also general information about the repository like the
       origin, codename or version number of the release.

       This information is shown in various places so a repository owner
       should always ensure correctness. Further more user configuration
       like apt_preferences(5) can depend and make use of this
       information. Since version 1.5 the user must therefore explicitly
       confirm changes to signal that the user is sufficiently prepared
       e.g. for the new major release of the distribution shipped in the
       repository (as e.g. indicated by the codename).

IMHO either the message from apt-get or at least the apt-secure(8) man
page should explain,

a) that (only) "apt update" (and neither apt-get nor aptitude) provides
   an interactive prompt to accept that change, and

b) that apt-get can accept that change via the
   --allow-releaseinfo-change option. (And yeah, aptitude lacks that
   option as of now.)

Had a user nearly going nuts because of not finding out how to fix this
due to not being mentioned in the message nor in the referenced man
page. (He didn't look into the apt-get man page as that one wasn't
referenced.)

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