as of v.2.9.4.3

add_filter('wppizza_custom_transaction_id', 'my_custom_transactionid_filter',10,2);
function my_custom_transactionid_filter($transactionId,$orderId){
/**prepend 1234 for example***/
$transactionId='1234-'.$transactionId;
return $transactionId;
}

NOTE: this filter will NOT write to or change entries in the database, it will only change the display of how you want to display the transaction id, so it will – of course – not retroactively change the transaction id in any emails already sent. It will however change the transaction id’s in emails that are send when this filter is active