Obama really hasn't changed his policy on gay marriage. He changed his rhetoric.
Before, he was "against" gay marriage, but took no action to deter it, actively ignoring DOMA.
Now, he's "for" it, taking no action to promote its broader enactment, supposedly leaving it to the states in an attempt to soften the news.
In both cases, Obama was and is relying on inaction to promote the broader legal acceptance of same sex marriage. A few more states will pass SSM laws, a few more might impose it judicially. And one day, perhaps not long from now, the Supreme Court will apply the Full Faith and Credit clause to SSM. And then we'll have national same sex marriage.
The eventual application of Full Faith and Credit is one of the few things left and right both agree will happen. The only way to head it off would be with a Constitutional amendment, and that's not likely to happen.
Obama's superficial change of position means absolutely nothing.