Board Thread:Support Requests - Community Management/@comment-3235402-20130404181023/@comment-5028213-20130404233346

You cannot alter the license,
 * 1) not of already licensed content, since the license is irrevocable (see 3. License Grant: "perpetual […] license"),
 * 2) not for derivative works; this is where the SA from CC-BY-SA kicks in ("share alike").