When removing a Page Workflow and adding a new one to a document, if the new workflow uses the same statuses as the old workflow and the workflow history was not deleted, the document will be automatically moved to the last state of the page with the old workflow.
That being said, if the page had an approval process running before changing the workflow with the new one, already assigned users won’t be changed automatically unless the page is transitioned, for example updating the page. That way, the approvers/reviewers will be refreshed with the new values added to the new workflow.
In the following example, the user ‘Ashley’ was automatically added as an approver in the Review status:
Now, If the already running workflow is modified with another approver, the page will need to be updated for the approval user to change.