When running "apt-get update" against a repository with a change in the
codename, apt-get outputs the following lines:
The apt-secure manpage documents the presence of this feature, but does
not provide any additional information about how to accept the change:
This is a bit frustrating to try to unravel, as the only documentation
referenced in the apt-get output doesn't actually tell you what to do.

I would suggest referring the user to the --allow-releaseinfo-change
flag in either the apt-get output or the apt-secure manpage.  Without
some kind of pointer in this direction, presumably the user would have
to read the apt-get manpage in its entirety to discover that this flag
exists.

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