Chiming in because I recently went through the upgrade process again.
No. It's probably not possible, and definitely not worth it. Unless you wanted to somehow go and rename all of the existing nGUI classes (for example UISprite-> UISprite_old), I guess. Even then, that's worrisome. Don't do that.
As far as upgrading the project: so many, many things have changed since 2.x, don't expect to be able to upgrade the project without nearly re-making every UI. Also, I'm not even sure the "upgrade from 2.x to 3.0" functionality is still bundled in the latest version of nGUI. I could be wrong though.
Back when 3.0 came out, we upgraded to it from 2.x. It took about two weeks to catch all of the big issues and resolve them, but even now I find things occasionally that were broken during that upgrade. And three weeks ago, we upgraded from 3.0.8 to 3.7.6. I'm still working overtime to fix issues / weird behavior.
If your deadline really is close, hold off and wait to upgrade until after you ship, if you can. I really recommend gritting and bearing it for now.
