When you want to change the reference of your invoices at the beginning of the new fiscal year, you must access from your profile to Settings > Basic settings > References > New.
Invoices > New.
You must indicate in document type, Ordinary invoice. In the same way you have to fill in the fields Name, Prefix. Optionally, you can indicate the number of digits, Title and Template.
Check the "Default" option to make it the default reference when generating invoices.