No worries - it might actually work out more expensive asking ANA to tag the Sapporo flights on by the time they reprice your tickets and if they charge a change fee.

The only thing it would do is protect your ticket in the event of any delays as the flights would all be on one booking.

Another option would be to take a later flight to Sapporo and leave your luggage at the airport and go into Tokyo for the day or get a day room at a hotel and then collect your luggage on return before your flight.