Build a report of all the records that are going to be affected by this change. Use the Search & Replace feature. Choose the checkbox field from the drop-down selection; and set the from: value to FALSE and the to: value to TRUE. Then click OK twice to update all the records. The first OK will tell you how many records will be updated by the change; and the second will action the update and it will then tell you how many records were updated in the modal before you close it manually.