Beyond 14 GPa of pressure, bilayered La_{3}Ni_{2}O_{7} was recently found to develop strong superconductivity above the liquid nitrogen boiling temperature. An immediate essential question is the pressure-induced qualitative change of electronic structure that enables the exciting high-temperature superconductivity. We investigate this timely question via a numerical multiscale derivation of effective many-body physics. At the atomic scale, we first clarify that the system has a strong charge transfer nature with itinerant carriers residing mainly in the in-plane oxygen between spin-1 Ni^{2+} ions. We then elucidate in electron-volt scale and sub-electron-volt scale the key physical effect of the applied pressure: it induces a cupratelike electronic structure via fractionalizing the Ni ionic spin from 1 to 1/2. This suggests a high-temperature superconductivity in La_{3}Ni_{2}O_{7} with microscopic mechanism and (d-wave) symmetry similar to that in the cuprates.