If I create the new ad in adsense and use it for sites or blogs. But after few days ago I replace the new ad of different size. So I want to leave the first ad. how can I leave the previous ad in adsense. Which action will be taken. Active ad hide it or leave it. Tell me in detail.
Im not sure I understand exactly what you are asking. If you created a 125x125 ad (for example) and later on wanted a different size ad for the same location, simply create another ad in Adsense and place the code where the old code was. Your old ad will still be in Adsense for later use.
The ads will remain in you Adsense dashboard, they will however become hidden once the system picks up that they are no longer in use. If you ever want to use the original advertisement again just go to the "My Ads" tab, click on "Additional Filters" and click on the "Hidden" check box to show your older ads.
yes, yuo don't have anything to do special, just create new ad unit and don't use the old one anymore, it'll work fine.