Fare rules may preclude what I'm about to say but the procedure to downgrade is:
Cancel and Make booking in new class.
Calculate the new total fare for the whole journey.
Compare the fare/total of the original ticket with the new fare/total and, assuming the new fare is lower, proceed as follows.
Reissue the original ticket (which means using it to pay for the new ticket). If the new total is less than the previous you show the new fare and appropriate taxes in their correct boxes and NO ADC (No Additional Collection) in the total. Then you issue an MCO (Miscellaneous Charges Order) or these days an MPD (Multi Purpose Document) or even VMPD (Virtual MPD) in exchange for the original ticket (again) and show the refundable value there (in words) but, to confuse everyone, the total of that document also shows NO ADC. The you refund the MCO/MPD/VMPD.
Job done!