Erik Moeller wrote in effect:
However, please keep in mind the one-way incompatibility that would result from adopting a non-copyleft license
A CC-by project could become a CC-by-sa project any time that it wished. If the incompatibilities between CC-by-sa and GNU FDL are worked out, then it could become a GNU FDL project.
This is not to say that any such change would go smoothly, because there would doubtless be resistance from some users to any change. ^_^
-- Toby