(This is not legal advice, but I do deal with software licenses professionally.)
> IANAL, but “GPLv2 or later” allows you (the recipient of the license) to choose either GPLv2 or GPLv3 (i.e. the one that’s more convenient to you) but does not allow you to prevent others (the recipients of your modified version) from having the same choice.
> This goes both ways, “GPLv2 or later” cannot be changed to either GPLv2 or GPLv3 only without permission from everybody that has ever contributed to the codebase.
This is not the case. A dual license allows you to redistribute under one or the other license, or both. "GPLv2 or later" must necessarily allow distributing under GPLv2 or GPLv3, because otherwise it wouldn't be compatible with GPLv2 or GPLv3. You're also free to combine it with GPLv2-only code (in which case you must distribute under GPLv2-only), or GPLv3-or-later code (in which case you must distribute under GPLv3-or-later).
> The FSF requires copyright attribution from cotributors, that’s why they were able to switch their projects to GPLv3-only.
The FSF requires copyright assignment, but that's not why; any project under GPLv2-or-later may be changed to GPLv3-or-later. And new code would then be contributed under GPLv3-or-later.
Also, the FSF doesn't like to use "GPLv3 only", for the same reason they don't like to use "GPLv2 only". FSF projects under GPLv2-or-later switched to GPLv3-or-later.
> IANAL, but “GPLv2 or later” allows you (the recipient of the license) to choose either GPLv2 or GPLv3 (i.e. the one that’s more convenient to you) but does not allow you to prevent others (the recipients of your modified version) from having the same choice. > This goes both ways, “GPLv2 or later” cannot be changed to either GPLv2 or GPLv3 only without permission from everybody that has ever contributed to the codebase.
This is not the case. A dual license allows you to redistribute under one or the other license, or both. "GPLv2 or later" must necessarily allow distributing under GPLv2 or GPLv3, because otherwise it wouldn't be compatible with GPLv2 or GPLv3. You're also free to combine it with GPLv2-only code (in which case you must distribute under GPLv2-only), or GPLv3-or-later code (in which case you must distribute under GPLv3-or-later).
> The FSF requires copyright attribution from cotributors, that’s why they were able to switch their projects to GPLv3-only.
The FSF requires copyright assignment, but that's not why; any project under GPLv2-or-later may be changed to GPLv3-or-later. And new code would then be contributed under GPLv3-or-later.
Also, the FSF doesn't like to use "GPLv3 only", for the same reason they don't like to use "GPLv2 only". FSF projects under GPLv2-or-later switched to GPLv3-or-later.