Is it possible to restructure classes during the middle of an accounting year and what is the result ?

Yes, it's possible to edit and add classes, mark-whitcombe. 

The Classes feature help you track which class or where a majority of your business income is coming from. Changing these information won't affect any accounts and your books. 

You'll need to manually open the transaction and either add the class per line item or for a transaction as whole. You can pull up a detailed or summarized report for your class tracking overview. 

Let me know if you need more help with managing your transactions.