Inside AdSense informs that you can now change AdSense layouts without making changes in your HTML/ JavaScript snippet.

The post writes,

"This new ad management feature means that your ad unit settings (such as colors and channels) for new AdSense for content ad units will be saved in your AdSense account every time you generate ad code. Then, if you'd like to change any of these settings in the future, all you do is make the update within your account — you'll no longer need to manually replace the ad code on all of your pages. For instance, you can quickly change the borders of all your 300×250 medium rectangles from red to blue with just a few mouse clicks. Fancy! We hope that this new feature will help you save time and will simplify the process of optimizing your ad units.

This feature, however is not available in any publisher accounts. This will be introduced in phases in the next few weeks. When you see the "Manage Ads" page appear under your AdSense Setup tab, you'll know the feature is available for you to use. Here's what it will look like:

For more information visit Inside AdSense.