Originally Posted by whlinder

My guess for this is that the GDS' recognize F and R as the same class. Officially (IATA rules) there are only 3 cabins, F, J and Y and there are no plans to add a 4th. R is a subset of the F cabin and I believe buying up to a higher cabin using a differential can only occur between different cabins. I don't think you can buy a differential to a higher booking code. The GDSs probably don't have the functionality (yet, if ever) to allow that.