Hi Rik Butcher,
When you hit the Add Filter button the filter won't know what FilterExpression to generate unless you specify it. Also, the filter does not now for which of the FieldNames to generate the filter, unless you specify it using code.
An extension that you can make to the previous solution is to handle the situation when the ChangeExpressionFieldName happens.
i.e
Regards,
Genti
the Telerik team