When you create an invoice the current value (the rate) for the tax is copied from the tax rate into the invoice and stored. Whenever you display a print preview or edit the invoice, the invoice uses the value that’s now stored within itself. You can change the original rate value in the definition of the tax rate, but what’s on the invoice stays the same. The reason why it does this is because tax rates change over time, but the invoice should stay at the amount that it was when it was calculated.
You’ll notice two textboxes next to the rates when you edit the invoice so that you can see what’s actually stored in the invoice. The current value is still shown in the dropdown list for the rate.
As an example, let's say we created an invoice for Orange County in Texas a year ago and, at the time, the rate was 2%. Recently, the rate changed to 2.1% which we’ve just updated in the definition of the tax rate. If we open up or preview the old invoice we should still see the rate that’s stored in the invoice (2%) like below. Any new invoices created after the rate has changed will use 2.1%. Note that the name will update no matter what, so if you change “Orange” to “Apple”, all invoices old and new will say “Apple”, but will have their stored rate.
You’ll find this behavior in other places throughout Quantify as well, the daily rent rate being one. You can update a rent rate without causing a change to the existing invoices.