2 weeks ago I flew LHR-LAX on CO codeshare with VS; I booked 3 days before travel and UK Onepass said there were no upgrade seats, but the silver elite desk said yes there were seats available, but only for a full Y fare, not any other upgradeable fare. The lowest economy fare was GBP250, K class was GBP750, but full Y was GBP2000. I don't understand why if the seats were available (3 days from departure)I couldn't use 40K miles to upgrade, not 20K (and neither did the OnePass agent nor supervisor, but there was no budging ). So I flew economy
To make matters worse, at LAX I was asked at check-in if I wanted to upgrade, but they would only accept a paper upgrade (of course did not have one as was on the cheapest fare and not expecting to be allowed to upgrade
BTW 747 in economy much better than A340