Ugrading room at check-in - group rate or rack rate?

We booked a Garden view regular room,
resort matched the current group rates for the week even though we booked direct.

I was wondering when someone books with a reduced rate and then wants to upgrade room type at check-in,
do they charge the difference at 'rack rate' or do they charge it still at 'group rate'?

thanks

I believe if you wait until check in you have to pay for difference in rack rate.