The new RadControls for ASP.NET AJAX have evolved quite a bit, compared to the old ASP.NET RadControls, which has led to improvements, changes and additions to their HTML and CSS code.
Instead of migrating your old skins, it will probably be a lot easier and time sparing for you to create them again, based on the latest embedded skins. You can also use the Visual Style Builder.
If you prefer to create the custom skins manually, then please refer to the skinning documentation of each control, for example
If you prefer to have a custom skin, which is based on an old ASP.NET AJAX skin, which is not available any more as embedded (such as the Q3 2008 skins), you can use the following skins as a starting point:
the Telerik team