The flowing steps can be used to "move" the unused open advance amount from one project to another.
You will be creating a total of three invoices, with the end result leaving Project A with no open advance amount. And then Project B now having that advance amount open.
1a. Create a revenue invoice for the advance amount open on Project A.
1b. Blank out the project on the invoice line.
1c. Note the sales account on the line.
1d. Apply the advance to the revenue invoice.
* Project A should now reflect there is no longer an Advance Amount Open.
2. Create an advance invoice for project B for the amount above.
* Project B should now reflect there is an Advance Amount Open.
3a. Create a credit memo invoice for the amount above.
3b. Blank out the project on the invoice line.
3c. Note the sales account on the line, it should be the same as Invoice 1.
3d. Apply to the advance invoice on Project B.
* This will wash out the amounts at the GL level. And the advance on Project B now shows as paid.