NGUI 2.3.1 is ancient at best. Upgrading to NGUI 3 is going to be difficult as it's a quite different system. You should use the NGUI 2 to 3 upgrade tools to make it easier, but it's still going to be a challenge. There are just too many differences between the two versions.
Your best bet is to upgrade to the latest in NGUI 2 first before moving to NGUI 3. Although if I was you, I'd just stick to the latest in NGUI 2 series, assuming it's a project that has already been shipped. An update to a shipped game is not worth upgrading the entire UI system over.