Switching between CPU and GPU rendering in Silverlight (XNA)

Hi, I have a project that uses Silverlight and XNA. I would like to make it run on older computers. I found out tutorial: http://www.riemers.net/eng/Tutorials...nce_device.php but i got error: 'Microsoft.Xna.Framework.Graphics.GraphicsAdapter' does not contain a definition for 'DefaultAdapter'. I tried to add a reference to other version of Microsoft.Xna.Framework.Graphics library, but I got an error that it's not possible to add a reference to non-SL library in SL project. What can I do if I need to switch between CPU and GPU rendering in Silverlight-XNA projects?

Thanks in advance.