Does anyone know if its possible to book and pay for a full fare Y ticket on SQ from SFO to HKG and then upgrade to J with Aeroplan?
Possible? Maybe, look at the rules for alliance upgrades using your miles. Having the full Y fare might be required. Likely? I would guess not because SQ has been notoriously ungenerous in releasing award space in premium cabins to its partners and even to its own FFers.