Opera 12.17 tries to upgrade to 25
(Post-warning)
Update: I emphasize that the point is to silently update the browser to a new branch, which Opera promised never to 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, now I have to use other browsers, including the new Opera.
As you know, Opera promised that version 12 would not make attempts to upgrade to next-generation versions. And, indeed, 12.15 was updated to 12.16, and that one was updated to 12.17.
Therefore, when Opera 12.17 asked me to upgrade today (without specifying which version) when I started it, I was glad that it was still somehow supported - and I would get 12.18.
I think it’s easy to imagine my surprise when I saw this window at the end of the process:

Therefore, be prepared to give up the temptation to update version 12.
If you still installed the new version, but did not restart the browser, my solution may help you to avoid the update.
I deleted three files from the Opera folder that are probably related 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 text like “Thank you for choosing the new Opera”, confirmed that opera is the default browser, and the native old opera started.
At the next restart, nothing unusual happened.
Perhaps (I hope), the new Opera is put parallel to the old one and at startup it asks whether to return to the old one, but I did not dare to check. In addition, my new one is already worth it, the installer could have checked it.
Be carefull.
Update: I emphasize that the point is to silently update the browser to a new branch, which Opera promised never to 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, now I have to use other browsers, including the new Opera.
As you know, Opera promised that version 12 would not make attempts to upgrade to next-generation versions. And, indeed, 12.15 was updated to 12.16, and that one was updated to 12.17.
Therefore, when Opera 12.17 asked me to upgrade today (without specifying which version) when I started it, I was glad that it was still somehow supported - and I would get 12.18.
I think it’s easy to imagine my surprise when I saw this window at the end of the process:

Therefore, be prepared to give up the temptation to update version 12.
If you still installed the new version, but did not restart the browser, my solution may help you to avoid the update.
I deleted three files from the Opera folder that are probably related 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 text like “Thank you for choosing the new Opera”, confirmed that opera is the default browser, and the native old opera started.
At the next restart, nothing unusual happened.
Perhaps (I hope), the new Opera is put parallel to the old one and at startup it asks whether to return to the old one, but I did not dare to check. In addition, my new one is already worth it, the installer could have checked it.
Be carefull.