An electric dipole consists of two small spheres with charges \(+q\) and \(-q\) connected by a rigid insulating rod of length \(d\). The dipole is initially aligned parallel to a uniform electric field of magnitude \(E\), with the positive charge downstream in the direction of the field. An external agent rotates the dipole by \(90^\circ\) about its center so that the rod becomes perpendicular to the electric field lines. How does the electric potential energy of the dipole-field system change, and why?