(Post-warning)
Update: I emphasize that the matter is in
silently updating the browser to the new branch, which Opera promised to
never do (update: pepelsbey says - they did not promise). This is a very unexpected behavior. The article is addressed to those who
consciously want to have 12.17, including in parallel with the new version.
Despite all the vicissitudes, my main browser is still Opera 12.17, although, of course, I now have to use other browsers, including the new Opera.
')
As you know, Opera promised that version 12 will not attempt to upgrade to the next generation version. And, indeed, 12.15 was updated to 12.16, and that - to 12.17.
Therefore, when Opera 12.17 offered to upgrade at launch today (without indicating which version it was), I was glad that it was somehow supported — and I would get 12.18.
I think it is easy to imagine my surprise when I saw a window at the end of the process:

Therefore - be prepared to give up the temptation to upgrade version 12.
If you did install the new version, but did not restart the browser, perhaps my solution will help you to avoid updating.
I deleted three files from the Opera folder, which probably belong to the new Opera:
launcher.exe
installer_prefs.json
installation_status.xml
After restarting the browser, I saw a window with a progress bar and a text like “Thank you for choosing a new Opera,” confirmed that opera is the default browser, and the native old opera was launched.
At the next restart, nothing unusual happened.
It is possible (I hope) that the new Opera is put parallel to the old one and asks if it is started up - if it is going back to the old one, but I did not dare to check. In addition, I already have a new one, the installer could check it out.
Be carefull.