Google said this will lead to a 5% increase in conversions, on average of all advertisers who switch. Google added:
To ensure that your campaign settings are also in line with your conversion goals, we plan to migrate existing campaigns using Conversion Optimizer or Enhanced CPC with "Optimize for clicks" ad rotation to "Optimize for conversions" ad rotation.
If you do not want to be migrated, tell Google by January 31st by using this form.
Here is a video with more details on this tool:
Forum discussion at WebmasterWorld.