I was on SQ620 (2 pax, Biz saver award) and it was dropped on our planned flight date and we were also moved to SQ622 losing the whole day, basically like the OP (reported in another thread about rebookings, anyways).

When I called in, as other posters advised, I was able to request a move to SQ618 and the agent (a well trained one that time) got a supervisor involved and they opened up redemption seats and moved us to that flight which had been our preferred choice from the start, but wasn't available for redemption back when making the booking.

So long story short, if you are not sure on your preferred flight, wait and SQ620 may actually come back and you'll be put back onto it. If you are keen on getting onto SQ618 instead, you can consider calling and using your situation to get the move pushed through as a free change. Good luck.