You may wire the SelectedIndexChanged event of the Combobox from either the design page or else from the code behind page. Set the AutoPostBack property of the RadComboBox to true. SelectedIndexChaned event will fire once you change the Selection of the RadComboBox.Then in the SelectedIndexChaned event you may access the GridEditForm item as shown below.
CS:
protectedvoid RadComboBox1_SelectedIndexChanged(object o, RadComboBoxSelectedIndexChangedEventArgs e)