The GPL "upgrade" clause,
> Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and "any later version", you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation.
> If the Program specifies a version number of this License which applies to it and "any later version",
I'm no lawyer, but I think the source has to mention that explicitly. So it's not true of ALL gpl projects. In this case though you are right because nano, it appears, specifies it's license as:
> License: GNU General Public License v3 or later
So, while true in this case it's not always true (see Linux kernel license, GPLv2, no upgrade).
»We promise to always keep the software free. This promise extends to any successors in the copyright, meaning that even if the FSF were to go away the freedom of all users to share in the contributions wouldn't.« — https://www.fsf.org/bulletin/2014/spring/copyright-assignmen...
This is the difference between copyright assignment to the FSF and most other copyright assignment.
Not to mention that much of the code inside Linux is under GPLv2-or-later, but because some of it is GPLv2-only the whole work is GPLv2-only.
Just do a grep for "at your option", since that's the wording in the copyright header to specify that it's released under GPLv2-or-later, tells me that it isn't that great. It's around 10000 files (Linux has ~50000 files) so it's ~20% of files. This doesn't account for documentation or files that don't specify that they are under GPLv2-or-later.