YouTube recently added a new feature in their backend which enabled users to set a default ad format for the new videos that would have monetization enabled.

The company is now planning to apply these changes to older videos with monetization enabled.

They have sending email alerts about this upcoming change to all the affected users. They are of course allowing you to opt out in case you do not want them to do so.

YouTube spoke about why it would be a good idea:

As part of our mission to help you to increase your revenue on YouTube, on Nov. 9, 2012 we will be using your default ad format settings to enable ad formats on your older monetized content.

If you have not enabled a format by default for new uploads, we won’t touch that setting on any video. This change will only enable ad formats on your monetized content; it won’t disable any formats you’ve set on older videos.

If you do not want to add these formats to your older videos, you can easily opt out at http://www.youtube.com/my_videos. If you wish to opt out, please do so by Nov. 9, 2012. After that date, we will add your default ad formats on all of your older monetized content.