The worst that could happen is that there is a future GPL version that removes copyleft. If there is a future GPL version that is horrible for the receipient, then at worst that receipient can simply opt for GPLv3.
Clause 14 of GPL 3 also clarifies what later licenses can do. They must be "similar in spirit" and they cannot impose any additional requirements on any author or copyright holder.
edit: Okay, after seeing clause 14, I don't even see how a judge would interpret a GPL version without copyleft to be "similar in spirit", so I can't even imagine how that would happen. I think you're ok in accepting newer versions of GPLv3.
I guess "Creative Commons Attribution-Share Alike 3.0" is probably similar in spirit to the GFDL 1.2, but there was some large amount of top-downess in using the or later clause to help Wikipedia switch licenses.
Wikipedia's content used to be licensed under GFDL 1.2 or any later version. In order to transition to a more appropriate license the Wikimedia Foundation asked the FSF to release a new version (GFDL 1.3) that specifically allowed relicensing to CC BY-SA.
The new license may be "similar in sprit" but what it allows relicensing to may not be.
That seems to contradict the clause that says no further obligations can be imposed upon authors or copyright holders. In particular, they cannot be forced to provide a warranty.
https://www.gnu.org/licenses/old-licenses/gpl-2.0-faq.html#V...
Clause 14 of GPL 3 also clarifies what later licenses can do. They must be "similar in spirit" and they cannot impose any additional requirements on any author or copyright holder.
edit: Okay, after seeing clause 14, I don't even see how a judge would interpret a GPL version without copyleft to be "similar in spirit", so I can't even imagine how that would happen. I think you're ok in accepting newer versions of GPLv3.