Upgrades using miles

I was told by the FC helpline that even when upgrades in exchange for FC miles may not be available in advance for some flights asking at the airport on the day of departure can more often show some available.
I've done that a few times now from NY to UK. Upgrading an S class ticket to Upper.
I tried yesterday for the VS046 and was told yes it's an S class ticket, but as it's an IT fare (via the company tarvel agent) it can't be upgraded using miles.
The FC helpline told me it could be if there were available upgrade seats released.
does anyone know which is right? The ticket was still showing an S class. I have bought tickets in the same way in the past and been able to upgrade.
I've done that a few times now from NY to UK. Upgrading an S class ticket to Upper.
I tried yesterday for the VS046 and was told yes it's an S class ticket, but as it's an IT fare (via the company tarvel agent) it can't be upgraded using miles.
The FC helpline told me it could be if there were available upgrade seats released.
does anyone know which is right? The ticket was still showing an S class. I have bought tickets in the same way in the past and been able to upgrade.