Before installing stuff on my new beowulf 3.0.0, I've run:
root@greipner:~# apt-get update
Get:1 http://deb.devuan.org/merged beowulf InRelease [33.2 kB]
Get:2 http://deb.devuan.org/merged beowulf-updates InRelease [26.1 kB]
Get:3 http://deb.devuan.org/merged beowulf-security InRelease [25.7
kB]
Reading package lists... Done     
E: Repository 'http://deb.devuan.org/merged beowulf InRelease' changed
its 'Suite' value from 'testing' to 'stable'
N: This must be accepted explicitly before updates for this repository
can be applied. See apt-secure(8) manpage for details.
E: Repository 'http://deb.devuan.org/merged beowulf-updates InRelease'
changed its 'Suite' value from 'testing-updates' to 'stable-updates'
N: This must be accepted explicitly before updates for this repository
can be applied. See apt-secure(8) manpage for details.
E: Repository 'http://deb.devuan.org/merged beowulf-security
InRelease' changed its 'Suite' value from 'testing-security' to
'stable-security'
N: This must be accepted explicitly before updates for this repository
can be applied. See apt-secure(8) manpage for details.

Unfortunately, the manpage only says:
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).

I.E. the secret method for explicit acceptance is not made explicit
AFAICT.
OK, there wasn't time to code the flinging up of a query for a [Y/n]
response in real time,but what on earth is the current work-around?
I'm quite content to try a dist upgrade, to 3.1.1 or whatever is the
state of the art, if thatcan safely be done with this brittle 3.0.0
version.

Erik