I'm guessing it won't be necessary to even worry about opting out. Historically Google delivers the best performing ads/types to your site that pay the best, since that benefits both parties (G and the publisher).
So if the CPA model is tried and shows it is not paying more than the CPC ads on your site, I'm guessing G will swap you back automatically. If you make more with the CPA model, more CPA ads will show. Etc.
Perhaps I'm too optimistic... =)