I know this is an older thread, but I wanted to share this anyway in case anyone is having difficulties with this.

You can't change the quantity itself, but there are a couple ways to work around this without having to go back and change the estimate. 

In the 'Due' column, you can choose to invoice for a percentage or amount of the total that was in the quote. 

 If you choose percentage, you can type any percentage of that number, and the quantity and amount columns will auto populate to reflect that change. For example, if you quoted a quantity of 1 at $100, you could invoice for a quantity of 2 at $100 by entering 200%. The quantity will change to 2 and the amount will change to $200.

  If you choose amount, you can enter what you need the total to be, and the quantity and amount columns will also change. Using the same numbers as the above example, if you quoted a quantity of 1 at $100 and need to invoice for 2 at $100, enter $200 as the amount and the quantity will change to 2.  

The rate column will remain the same using both of these methods, the only things that will change as you alter the 'Due' column are the quantity and amount. It will work if you need to invoice for a lower or higher quantity than you quoted.