This one time, at band camp, Dominic Hargreaves wrote:
In the case if this licence, if it's made sufficiently close to by-sa I think you could argue that the change was functionally equivalent to the existing setup.
It looks, from the discussion, that it's going to just be an optional tweak on by-sa, which might resolve the problem.
The GPL allows licensees to use the version of the licence on the work or any later version. by-sa 1.0 doesn't seem to have this, but 2.0 does seem to have it.
This entails quite a bit of trust in the organisation that writes the legal stuff, though the licence details seem to force that down a bit.
"under the terms of this License, a later version of this License with the same License Elements as this License, or a Creative Commons iCommons license that contains the same License Elements as this License"
http://creativecommons.org/licenses/by-sa/2.0/legalcode