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The logic for being able to re-assign an Approval is based on the 'PR Admin Review/Approval' Permission, which is part of the PR Administrator role. If the user has this Permission for the given PR then they will be able to change the assigned Approver for 'Pending' Approvals.
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Once the PR is assigned to someone else, the original approver, will become one of the possible approvers in the list, in the case the PR has to be reassigned to another approver again.
Purchase Requisition Administrator will find the PR under Approvals & Workflow/Review/Approval List and open it. In the Workflow/Messages screen, the approver name should be a drop-down which will show the available alternative approvers and their $limit. A new save button should be added to the screen.
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