Replace approver

Swap one approver in an approver group for another user without rebuilding the rest of the approval flow — the V3 replacement for V1 PUT /v1/approval_flows/{id}/approver_groups/{id}/replace_approver. The remove_user_id must currently be an unresolved approver on this group; resolved groups (already approved or rejected) cannot be modified. The add_user_id must be a different user, must not already be an approver anywhere else in the same approval flow, and must have read access to the underlying job or offer. To replace the entire set of groups on a flow, use PUT /v3/approval_flows/{id}/replace_approver_groups instead.