I've got an interesting question, that perhaps only a travel agent could answer. A large company I work with will hopefully be flying me on an international trip in J later this year, but I suspect they won't be booking the tickets for a month or so. There is nothing I can do to speed that up and one day I will get an email from their corporate travel agent saying here are your proposed flights, are they OK, would you like to change them. It is possible to change the flights and I've had colleagues who pay their own cash to upgrade the company purchased J fare to F. If I bought a voucher now, would it even work to ask the corporate travel agent to see if I could pay to upgrade to F and have them start the booking with my voucher and use my DSC period dated voucher PNR ? (Knowing that there are no guarantees that the voucher would attract DSC, but worth a try)
The other question I'm still not sure of, is it you buy an F class fare from SYD - LHR for example, would the DSC be at F rates or are the double status credits at the flexible economy rates?