I was using YUI 2.9 version before and now to upgrade it to YUI 3.18.1 what are the benefits that I will be getting from it. From where can I get those things which got improvised with YUI up-gradation.
None of them are projects in active development, you would be switching from on dead-end to another. GitHub shows the last change to its repository dated 3 years ago.
The changes to your code when migrating are substantial. There is no easy upgrade. There are no major gains in doing an upgrade. Not all YUI2 components are available in YUI3. The rewrite would be so extensive that you might as well use your time rewriting the full app to use some other technology.