I know that Adsense team read this forum and i would like to suggest something for future regarding message about
Recommendations Replacing.

It is great information's but there is very little information's what exactly need to be changed

10%-20% predicted increase in earnings.
Ad unit "336x280, created 10/05/11" is size 336x280. Our tests show you're likely to earn more if you replace it with a 300x250 ad unit

It is all. Very often i have created on same day maybe 10 different Ad unit "336x280" on 10 different pages. Or even more on different website.

So which one need to be replaced?

It will be great if there are more information's about replacing in future.