Pradeep, my question here is will the pricing schema remain same as current and you'll add new step for your particular condition type, if so then unless or until user will not update condition type in sales order change transaction or billing creation screen then it'll not appear while billing.
To avoid this condition update either control it via exit or ask business to bill the invoices for old sale orders now your job would be to create a copy of your existing pricing procedure by adding your new condition type in it and do the assignment accordingly for pricing schema determination, here from cut off date for new orders new pricing schema will be determined and for old orders pricing schema will be the old one. This solution will help you for better reporting.