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When upgrading to a new release of Yii, you do not need to clean up the folders under `assets/`. Yii will automatically re-publish assets to new folders based on the Yii version number. The old folders will remain there and unused.
If you are using the SVN development branch of Yii and find any asset file updated in the Yii framework after SVN update (e.g. `jquery.yiigridview.js` is changed due to a bug fix), you should clean up your `assets` folder so that the updated asset files can be re-published.