If the SJC actually reformulated the common law meaning of marriage, does that affect the statute?
No. Common law marriage has never been allowed in Massachusetts. Even if the SJC could change the common law definition of marriage (putting aside the legislative aspects of that action), the common law is not the highest but the lowest form of law and, therefore, is subordinate to legislation enacted by the representatives of the people (i.e., the statute supercedes the common law, rendering that common law redefinition ineffective).