[bobjam]

It is conceivable they could uphold the ban (afterall, how can a constitutional amendment be unconstitutional) but refuse to void the existing “marriages” on the grounds that voiding them under Proposition 8 would violate the ex post facto provision in the US Constitution.

[freedumb2003]

It is conceivable they could uphold the ban (afterall, how can a constitutional amendment be unconstitutional) but refuse to void the existing “marriages” on the grounds that voiding them under Proposition 8 would violate the ex post facto provision in the US Constitution.

That seems to make sense. In the same way that the "revision" vs "amendment" argument is uphill, trying to end those marriages would also be uphill.

[TeleStraightShooter]

It is conceivable they could uphold the ban definition of marriage (afterall, how can a constitutional amendment be unconstitutional) but refuse to void the existing “marriages” on the grounds that voiding them under Proposition 8 would violate the ex post facto provision in the US Constitution.

The SCOTUS dissents with your opinion:
...in the early case of Calder v. Bull(1798), the Supreme Court decided that the phrase, as used in the Constitution, applied only to penal and criminal statutes.

Notwithstanding the fact this is a civil code question, an interesting question is raised. Art I, Section 9's Ex Post Facto clause applies to the Legislative branch of the Federal government, not the Judicial branch since they do not write laws, right?

When the Judicial branch ventures past the separation of powers delineated in the USC into the Legislative powers do such prohibition in Sec 9 apply to the Judicial branch.