As obvious as that sounds, it's not the role that attorneys general have commonly played; historically, the official legal representative for the state or nation defends newly passed laws against court challenges. Nor is this an easily resolved constitutional issue.
Certainly, Proposition 8 is not a wholesale rewrite of the state Constitution. The question is whether fundamental change has to mean extensive and elaborate change, or whether it's enough that a ban on gay marriage deprives a group that has long been a target of discrimination of an essential constitutional right.
The question deserves a well-considered answer by the court -- and an honestly rendered argument from Brown. If Brown deems Proposition 8 an unconstitutional revision, the state deserves better than a pro-forma argument defending it simply because that's what his office has usually done -- just as Brown should defend the measure vigorously if, in his opinion, it is an amendment, even though he opposed it before the election.
Though it's the less common path, there are precedents in which attorneys general have taken a stance on constitutional grounds, rather than on the idea that they must defend the will of the voters no matter their own legal beliefs. In a 1983 case decided by the U.S. Supreme Court, U.S. Atty. Gen. William French Smith sided against a law that would have allowed a single house of Congress to overrule immigration decisions. In 1967, California Atty. Gen. Thomas C. Lynch argued against Proposition 14, a rollback of fair-housing laws that was ultimately found unconstitutional.
Californians on both sides of the gay-marriage debate deserve an equally principled argument from Brown.