The sustainable recycling and reprocessing of Nd-Fe-B magnets are essential for reducing critical-material dependencyin next-generation permanent-magnet manufacturing. This study introduces a single-grain, grainboundary-engineering strategy that revitalizes RE2Fe14B grains recovered and extracted via chemical leachingfrom end-of-life (EoL) wind-turbine magnets. The study showed that the selective leaching disrupted the matrixgrains, producing Nd-, Dy-, and Fe-based oxides that shaped the subsequent microstructural evolution. A detailedtransmission-electron-microscopy (TEM) study was employed to track the chemical and structural evolution ofthe modified grain boundaries and to correlate these changes with microstructural and magnetic performance.Nd70Cu30 additions proved decisive in dissolving RE2Fe14B-based surface oxides into the liquid phase and reprecipitatingthem at the triple pockets as oxygen-rich secondary phases, enabling surface reconstruction andliquid-phase sintering. The magnetic properties of bulk Nd-Fe-B magnet samples improved markedly with Nd-Cuadditions (0–30 wt%): the remanence increased to ≈ 1.05 T, saturating above 5 wt% Nd-Cu due to enhancedgrain alignment. The coercivity rose continuously from 50 to 825 kA/m with increasing Nd-Cu, governed primarilyby grain-boundary characteristics rather than grain size. The maximum energy product also increasedfrom 10 to 195 kJ/m3 under the same additions. Simulations showed that the effect of in-plane anisotropy due tothe presence of RE-oxides at the surface of the Nd2Fe14B grains reduces the coercivity. On the other hand, thebetter grain alignment markedly enhances the coercivity. The low-Fe-concentration grain boundaries, consideredto be paramagnetic, act as an effective magnetic decoupling phase. Conversely, excessive non-magnetic secondaryphases in the triple pockets generate local demagnetizing fields that lower the coercivity. Thus, optimizingthe oxygen pathways with Nd-Cu additions enables the sustainable recycling and reprocessing of Nd-Fe-Bmagnets by replacing the Nd-rich phases with resource-efficient Nd70Cu30. The study demonstrates the potentialof microstructural single-grain, grain-boundary re-engineering to enhance the properties of recycled magnets.