Earlier quoted context omitted.
The problem is if the maintainer wants to change the license (Like GPL2 -> GPL3). That's only possible if the contributors have signed over copyright, or the maintainer can track down every person who ever sent a patch, and get them to agree.
I don't think GPL2 -> GPL3 is a valid example of that. Usually it is to dual-license with a non-open-source license or some other incompatible open-source licenses. 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 t…
> 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).