Answer Re: Does the "GPL version choice" impact GPL-compatibility?

[Nathanael Nerode <neroden@fastmail.fm>]

Glenn Maynard asked:
>Is it valid to combine GPL work placed under "GPL version 2" with one
>under "GPL version 2 or any later version"?  That is, do versioning
>choices impact compatibility (when the versions overlap)?  Are all future
>modifications bound to give the same permission to upgrade the GPL?

Yes, it's valid to combine them.  The combined work, however, is effectively 
GPL-version-2-only.  No, future modifications do not have to give upgrade 
permission.  Ever -- you can take a "version 2 or later" program and put your 
modifications under "version 2 only".  (In fact, you can take a "version 1 or 
later" program and put your modifications under "version 2 only", or under 
"version 1 only".)

This is pretty straightforward; the "version 2 or later" is a form of 
dual-licensing (dual-licensed under version 2, version 3, etc.).
You pick which license you want to use.  If you combine a work dual-licensed 
under version 2 and version 3 with a work licensed only under version 2, you 
have to use version 2.