I'm no lawyer, but that situation -- grandfathered in same sex marriages are allowed to stand while new ones are prohibited -- seems a good basis to challenge, too
I agree, but I think the biggest near term effect is that allowing the existing marriages to stand just makes the whole argument for the
necessity of Prop 8 patently absurd. With each passing day a few dozen or a few hundred voters will see it that way, a few dozen or a few hundred young people who already see it that way will come of voting age, and a few dozen or a few hundred Prop 8 supporters in the older generation will pass on.
Long before the national climate & the makeup of the SCOTUS changes sufficiently*, CA voters will overturn Prop 8. It might be one year or three, but I doubt it will be five.
*The SCOTUS didn't rule that laws prohibiting Interracial marriage were unconstitutional until 20 yrs after the first state eliminated them (which was CA), and until after 2/3 of the states had followed suit. For same-sex marriage CA was first, and then it wasn't, and now it won't be, but I still reckon we will be among the first ten.