[elsjaako]

The standard GPL includes a clause that let's you published a modified version of the software under a later version of the GPL.

So if you write something under the GPLv2, and I change it and publish it, then the GPL says I need to release the source under the GPLv2 or later. This means that I can publish it under the GPLv3, and you can't take back my changes unless you upgrade your software to the GPLv3.

Linux removed the "or later" text from their version of the GPLv2, meaning that I can't change the Linux kernel and publish the result under the GPLv3.

[the_why_of_y]

This is incorrect, no version of the GPL has such a clause. The FSF merely recommends that authors give downstreams the option to use GPL version N "or later".

However, you can "upgrade" LGPL code to GPL, see LGPLv3 "2. Conveying Modified Versions".

There is a FLOSS license that has an implicit upgrade to new version clause: the Mozilla Public License, see "6.2. Effect of New Versions" in MPLv1.1.

[elsjaako]

I learned something today.

It looks like the "or later clause" is a built in option, but not a built in default, of the GPLv2. With that I mean that the meaning of that phrase is explained in the 9th clause.