You've put your finger exactly on the issue that determines whether you should copy (look up) data from a related table or simply link to it dynamically. When you copy the data, subsequent updates in the original table don't update records that looked up that value--a useful feature when you are looking up prices for items and need to be able to change the price for future orders without it affecting the price on past orders. But when you dynamically link to the data, any change in the original record automatically updates for you.
In this case, I'd just add the FY (fiscal year, I presume?) field from Orders to your invoices layout. The relationship you describe should allow you to add any fields from orders that you need to see on your invoice layout just be adding the fields directly to your layout. (I am assuming either just one order record for every invoice record or that all order records should show the same FY value.)