There are at least one very important thing you forget. The words "requests" in the original posting, and that a license is a binding contract in a large number of countries, will create a situation whereby one part changes the contract. If you change it in such a way as to break the original meaning, the new version is not binding any more. The question is not that it is possible to do the transition in some jurisdiction but rather if it will be binding for the contributors as such.
Herein lies the key to this whole issue: whether the contract is changed in such as way "as to break the original meaning". You're making the false assumption here that the spirit of the GFDL V1.2 will be violated by any migration, or that the CC-BY-SA is sufficiently different in meaning and spirit to represent a "break". This is simply not going to be the case. If I may quote section 10 of the GFDL:
"The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns."
The FSF has already included the possibility of new license versions with a similar spirit, but different details. Using the "any later version" clause, our contributors have already implicitly agreed to any future migrations from GFDL V1.2 to GFDL Vx.x, so long as the license is published by the FSF, and is similar in spirit (although different, ostensibly improved, in detail). Having a clause in the GFDL that allows a similar migration to another license with similar spirit but different details is already written into the contract. Whether we call this new license "GFDL" or not is, presumably, one such "detail".
My worse nightmare is a scenario whereby contributors claim Foundation has broken the license contract and goes after some firm using material from a Wikimedia project in some other jurisdiction.
If you think anybody is jumping into this without proper consideration, input from lawyers and experts, etc, then you must be terribly mistaken. All parties involved have a vested interest to insure that every possibly precaution is taken, and that all serious questions are answered before making any kinds of a move like this. The FSF has a lot more to worry about then the happiness of Wikimedians, and they certainly aren't going to put themselves in harms way any more then we intend to put ourselves there.
--Andrew Whitworth