Simplest way changing the dynamically item source is like this,
this.YourGridName.ItemsSource = //here assign your item source.
a similar post i have seen here , it will help you see this
http://www.telerik.com/community/forums/silverlight/gridview/itemsource-changes-radgrid-doesnt-update.aspx
Let me know if it is not helpful.
Regards
D