You are able to undo any changes you make on the Scheduler when it is still in draft mode. In the top right corner, you will see a button called 'Undo'. If you have no changes to approve, it'll be greyed out.
As you make changes to the schedule and they turn into draft shifts, the undo button will start showing you what changes you're making and how long ago these were made. As this happens, the approve button continues to count up.
If you realise you have made a mistake when changing a shift, you just need to click the undo button and it will remove the most recent change you made and revert it back to the shift it was before (and in an approved mode, meaning you do not need to re-approve this shift.)
If you undo all shifts, there will be nothing to approve. If you just undo the most recent, you will still need to approve the rest of the changes you have made on the Scheduler.
The undo feature also works on the multi-week view.