Flex :: Make An AdvancedDataGrid Re-display Its Labels When The Results Of The LabelFunction Change?

I have an AdvancedDataGrid with a custom label function whose value can change based on other form status (specifically, there's a drop down to choose the time display format for some columns).Right now, I have this labelFunction:

internal function formatColumnTime(item: Object, column: AdvancedDataGridColumn): String {
var seconds: Number = item[column.dataField];
return timeFormat.selectedItem.labelFunction(seconds);
}

[code]....

... and the timeFormat object is a combo box with items whose labelFunction attributes are formatTimeAsFractions and formatTimeAsInterval.The columns that have time formats have formatColumnTime as their labelFunction value, because extracting the seconds in that function and passing it in to the formatters made for a more testable app (IMHO).

when the timeFormat.selectedItem value changes, I want to force my grid to re-calculate the labels of these colums. What method must I call on it? invalidateProperties() didn't work, so that's out.