Estimate - Change Tax Codes En Masse

I'm trying to use a script to change the tax code for all the line items of an estimate. For testing I'm just using a button with the following code.

I know the script is running (due to the alert) but the value of the field is not changing and I'm not getting an error.

Can we even change the taxcode field?

function FlipTaxes()
{
var itemcount = nlapiGetLineItemCount('item') ;

for (var x = 1; x <= itemcount; x++)
{
  //alert(x);
  nlapiSetLineItemValue('item', 'taxcode', x, 'G6');
}


}

Thanks