Modifying SMPS power supply to produce a negative voltage
You need to understand how flyback circuits work. Just reversing the diode doesn't help. That blocks power transfer.
Power is transferred during the switch off time.
You need to put the rectifier diode at the other end of the secondary (the finish) in reverse orientation (same direction of current flow as at present). Then it'll work.
But why ? The output is isolated anyway. You decide what is negative and what is positive. Just reverse the output terminals !