We analyze the SOHO/MDI high-cadence magnetic field variations that occurred in a bipolar area around the apparent neutral line (ANL) prior to and during the 2B/X4.8 flare on 23 July 2002 and their association with the hard X-ray (HXR) and Hα emission observed with RHESSI and BBSO. Magnetic field changes started 6 min prior to the hard X-ray emission onset in 0.3--2 MeV band or 7 min in the 12--25 keV band and finished 20 min later. Before the flare, there are nine preexisting sources of a strong magnetic field of the positive and negative polarities, and six new magnetic sources associated with the hard X-ray foot points emerged 2 min before the flare maximum: two negative and four positive ones. The difference magnetograms show a steady movement with a speed of 250 km/s of the magnetic field changes toward the northeast, or a 'negative magnetic field discharge' associated with the magnetic energy release in this flare. The magnetic field changes in the whole flaring area are irreversible, showing an increase of the magnetic flux from 6.7 ¿ 1021 Mx up to 7.9 ¿ 1021 Mx lasting for 6 min and returning after to noise fluctuations about the new magnitude. There is an irreversible increase by -120 G of an absolute magnitude of the negative magnetic field component. The rate of magnetic field changes deduced from the total area around the ANL and from the areas of the hard X-ray foot point with the irreversible changes is about 3.1 ¿ 1018 Mx/s, and the average Poynting flux is (5--20) ¿ 1011 erg/cm2/s. The LOS fraction of a magnetic energy released for the whole flare duration is estimated as ~(2.3--10) ¿ 1031 erg. The possible scenarios of particle acceleration in this flare are proposed based on HXR and γ-ray emission.