My boss just informed me that he wants to begin making transition plans to get me ready to be the successor for an opening on our team where I will have increased responsibility for managing people and budgets. He wants to begin putting in place a transition plan to have me step into the new role sometime in Q4 14, but the transition plan is not final. When and how do I bring up the issue of compensation that comes with the new role?

Until you've got something...
...you've got nothing. Right now you don't have the new role -- you have someone saying he'd like to work on a plan to help get you ready.
The right time to discuss compensation changes for moving into a new role is when you move into the role.
flexiblefine
Houston, Texas, USA
DiSC: 1476