Normally, if you make changes to your approval workflow, they will only be applied to new requests.
However, it is possible to apply new workflow settings to already created but not yet approved requests by clicking on Start over the workflow.
You can do this either from the actual request:
or from the workflow settings:
Tick the requests you want to start anew and click on RESTART:
If a request has been started again, this will be mentioned in its audit trail at the bottom:
Without a restart, any already created but not yet approved request will have the 'historic' Approvers, who had been selected according to the workflow rules before they were changed.