Haven't fans always joked that Riku's hair is a measurement of the Darkness in his heart. Him having short hair seems like a reflection of that. When he was a kid his hair was short, so he didn't have that much Darkness. Then when he was 15, it was longer and he seemed to have a good amount of Darkness. In Days/KH2 it was uber long and that was when he started accepting more of the Darkness until he just became Ansem: SoD. Also, even after he returned to his regular form, he used more Darkness based attacks.
It looks like in 3D, he is using more Light-attibuted abilities and spells as well. His slashes are both light and darkness (like when he does a combo finisher), and he used Blizzard and Cure for the first time...
I rather like the new hair and I hope he keeps the style for the rest of the series. And if it starts getting long again, he should put it in a high ponytail.