As stated above, when you work with something like LESS, SASS or any other CSS preprocessor, you don't want to go back to plain old CSS. Working with LESS is just a joy, especially when you start realizing that copying styled elements is much easier, that variables are making your life much more fun when the customer wants to change the colours once again and much more.
That's why we don't want to go back to CSS either, but like most people, we have to work with old code from the days before CSS preprocessors and that just isn't t as much fun, especially if you know how good things can be. This is where CSS2LESS.NET might help you a bit. It won't create perfect LESS code, but it gives you a good start!
Want to help? This library is open-souce and could always use some help, checkout the github site: https://github.com/ortic/css2less