However, it is possible to apply new workflow settings to already created but not yet approved requests by clicking on 'Start over'.
You can do this from the request:
Or from the workflow settings:
Tick the requests you want to start afresh and click on the 'Start over' button:
If a request was started again, you will see it in its history at the bottom.
Otherwise, already created but not yet approved requests will have the 'historic' Approvers (if they were created before changes to the approval workflow were made).