If you don’t want to have your campaigns migrated, then tell Google by the end of January using this form.
According to Google it will lead to about 5% increases in conversion volume on average across all advertisers who switch.
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 want to keep track of the forum discussion, check out WebmasterWorld.
They posted a video with more details on this tool: