The online check-in can be easily enabled for the existing bookings. You just need to make sure that the guest receives an email which will contain a special key {ManageBookingLink}. This key will make that the guest will receive a message with a unique link which will let him perform the online check-in from his online guest account.

Please have a look here in order to see how to place correctly the mentioned key in the email template: How to send the information to guests about online check-in?