You need to edit the control template of RadDatePicker. Generate the template from Expression Blend, inside you will find the ContentControl x:Name="DropDownIcon", which is responsible for the icon of the drop down button. The template of this ContentControl is set as a StaticResource - ControlTemplate x:Key="CalendarIcon". Modify that in order to change the icon of the drop down button in RadDatePicker.
I hope this helps.
All the best,
the Telerik team
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