Depending on how far in advance the original ticket was booked, she/he may face the situation that there are no tickets available at the price she/he originally booked?

It fair to say, that inflight (f*****g) voucher is not a fair compensation for a downgrade. It should be partial refund of the ticket. SQ should work on that. They know its cheaper to refund with vouchers, as the real cost of the 150sgd/250sgd is just 60 to 70% provided the voucher is fully redeemed. If you consider that a lot of vouchers are not redeemed, the cost is even less. Cheapo magician trick.