My pleasure Koala. Happy to help!

It is advisable to migrate at the beginning of a FY. This is to keep all your financial data in one place from the beginning of a FY so the financial reports and tax filing can be as accurate as possible by avoiding overlap (or gap) of your financial records. 

Hope this helps. 

QBO supports import style where you can create an invoice template with Microsoft Word and then import it into QBO. Maybe you can start your work on the Word document with the invoice template before the migration completes? 

Please let us know if you have any questions. Thank you. :-)