Licensing: CC-BY-SA to CC-BY-NC-SA

Disclaimer: I am not a lawyer, and I don't expect anyone answering this question to also be, and I will not take the answer as legal advice.

Starting from a piece of software (in this case, a game mod) that is licensed as CC-BY-SA, can I create a derivative code (another mod, plus art assets) and publish my software as CC-BY-NC-SA? Or does the "SA" part in the original mod forbid this?

This is for a game that does allow mods, and which both licenses are already acceptable on the community.

I tried looking thru the licensing guidelines at the CC site, but I really haven't found a description that lets me know how can I sub-license or re-license content that is already CC licensed in a DIFFERENT license.