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In this work, we focus on the modification of the Cu element in commercial N40M magnets toward the composition and structure of the grain boundary phase. The magnetic properties and microstructure of Ce-magnets with low and high Cu content were investigated. The Ce-magnet with high Cu-content has higher coercivity and a more continuous and homogenous grain boundary phase. The grain boundary phase’s structural shift is found to be the source of the coercivity change. The transmission electron microscopy study shows that the PrNd23Ce7Gd1.5FebalB0.93Cu0.35M1 magnet contains fcc and hcp type (Ce, Nd)-oxide phases, each with a different O concentration. The formation of the superlattice has been examined, and the superlattice spots show that the Cu-rich phases have an ordered structure.