If you pay the fare difference to reticket from a J fare to an F fare you get everything that the F ticket includes. But if you accept a promotional upgrade offer (paid with either cash or avios) you will most likely retain the lounge access, baggage allowance and points earnings of your original fare basis. You need to read the T&Cs of the specfic "offer" you are buying very carefully if the extra F privileges are important (especially if you don't already have QR Platinum for AS access).