[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.