Toggle whether an existing approval flow is sequential (approver groups resolve one at a time in sort_order) or parallel (all groups activate at once). The flow must be in pending status — once it is approved or rejected the request returns 422. To restructure the groups themselves use PUT /v3/approval_flows/{id}/replace_approver_groups; to swap one user within a group use PUT /v3/approver_groups/{id}/replace_approver.