Air New Zealand Air Points - Upgrading fare basis from G to T?

Hi,

I'm looking to fly from LHR to HKG in October and have found a cheap G class fare. However, it order to earn frequent flier miles from on my Star Alliance card (BMI) I need a T class fare. Has anyone experienced paying to upgrade the ticket? Is it better to do this at the time of booking or at the airport? Can it be done for tickets booked outside of Airnewzealand.com?

Thanks

Jon

The cost is the fare difference to T plus a change fee. May as well book T directly.