I tried upgrading just the LHR -> ATH leg and I was quoted 17,000 Avios + $300 odd. However, if I look at the IATA Matrix website, there is no YQ quote for LHR -> ATH. Therefore, given I am not upgrading SFO -> LHR and just upgrading LHR -> ATH, shouldn't the charges be just 17,000 Avios and GBP 30 odd?