Definitely worth asking at the ticket desk. Earlier this year I asked at LHR about using miles to upgrade my N ticket (cheapest economy) to UC. In theory these tickets are non-upgradeable but I paid a bit more cash to convert it to an upgradeable economy ticket and then was allowed to use my miles. Note I only upgraded the return (PVG-LHR) but seemed to be more flexible asking for this in London than in Shanghai.