can Wikipedia apply "CC-BY-SA-compatible FDL" retroactively?

I read at http://creativecommons.org/weblog/entry/7876 about the 
agreement to modify the FDL to make it compatible with CC-BY-SA, so 
that Wikipedia articles can be republished under CC-BY-SA.  However, 
I was confused about two things:

1) I thought that the GFDL was already compatible with CC-BY-SA 3.0, 
since they both required derivative works to be published under the 
same license.  Is there a specific part where they're incompatible, 
or is it just a case that there are ambiguities about compatibility, 
and the FDL will be revised to remove all doubt?

2) More confusingly, I don't see how you can just "update" a license 
and retroactively apply it to all existing content that had been 
published under an existing license.  All the contributors to 
Wikipedia, for example, agreed to the terms of the old FDL when they 
submitted their work.  How can the updated FDL be said to apply to 
that work if the authors didn't agree to it?

I'm writing an article about Google Knol for Slashdot, about how they 
currently allow only CC-BY and CC-NC-BY license options, and arguing 
that they should allow CC-BY-SA as an option as well, allowing people 
to copy content from Wikipedia to Knol.  I argued before when Knol 
was first announced:
http://slashdot.org/articles/08/02/15/177258.shtml
that it would be a good idea to have what would essentially be a fork 
of Wikipedia where articles could be locked against editing and 
signed off on by credentialed experts.

	-Bennett

[email protected]     http://www.peacefire.org
(425) 497 9002