It's easy to modify the order number shown on the invoice so that it includes the year of the order.
For example, if you have order number xyz, ordered in 2008 and want to display it on the invoice as Invoice No. xyz-2008 you can edit the file admin/invoices.php...
find this line of code at about line 141:
PHP Code:
<td class="main"><b><?php echo ENTRY_ORDER_ID . $oID; ?></b></td>
and replace it with this:
PHP Code:
<td class="main"><b><?php echo ENTRY_ORDER_ID . $oID . '-' . date(Y,strtotime($order->info['date_purchased'])) ; ?></b></td>
Then all your invoice numbers will include the year of the order, without ever having to update anything manually.