> One of these [scenarios for relicensing GPLed softwar] is relicensing to a later version of the GPL. This scenario is in most cases already covered by the default "or later" language that is usually applied to software licensed under the GPL.
That language doesn't actually cover the case in question. The "or later" clause lets the licensee choose which version of the GPL they want, but the FSF's interest in relicensing to a later version is often that it wants to impose the stricter terms of a newer GPL version on licensees. A licensee could avoid the GPLv3 patent restrictions on software licensed "GPLv2 or later," for example, by choosing to stick with the GPLv2.
... unless he want to use a newer version of the software which RMS has decided should be licensed under the GPLv3 only. FreeBSD has run into this problem with gcc, for example: We could no longer backport bug fixes, but instead had to "clean-room" re-develop them.