is there a way to chage the LegendItemStyle when Legend Items are autogenerated?
I need to customize the label of items, but leave the look of items as is was.
Yes, you can style the Legend anyway you want to. You'll need to open your project in Expression Blend and extract the Legend template in there. Once the template is extracted, you can style it and set the instance of RadChart to use the StaticResource
This link will bring you to the documentation on how to do that.