I'd been waitlisted for ~3mths for a HND-SIN J seat (confirmed in PE) and it was still waitlisted when I was checking in for my flight yesterday. At the check-in counter they asked if I would still like to change my seat and I said yes. The manager called the SQ SIN office to confirm the additional mileage and change fee, and I got my ticket sorted out in about 10-15mins.