Magnetic properties of lanthanide endohedral metallofullerenes are strongly modulated by intramolecular metal–metal interactions, which suppress the quantum tunneling of magnetization (QTM) in Dy₂ScN@C₈₀, but lead to magnetic frustration with pronounced QTM in Dy₃N@C₈₀. In this work, we explore how exohedral chemical modification of Dy₂ScN@C₈₀ and Dy₃N@C₈₀ by photochemical addition of adamantylidene (Ad) affects Dy···Dy interactions and influences their single-molecule magnetism. For each fullerene, the photochemical reaction with adamantane aziridine produced two isomers of Ad monoadduct, minor [5,6]-open and major [6,6]-open. By virtue of the high sensitivity of the ¹H nuclear spin probe in the Ad moiety to the position of Dy ions, paramagnetic NMR helped to establish Sc-Ad coordination in the [5,6] isomer and predominant Dy-Ad coordination in the [6,6] isomer of Dy₂ScN@C₈₀(Ad). SQUID magnetometry and relaxation measurements demonstrated that Ad addition has almost no effect on the strength of the Dy···Dy coupling in the [6,6] isomer of Dy₂ScN@C₈₀(Ad), but it does increase the coupling in the [5,6] counterpart by 20%. The blocking temperature of magnetization and the coercivity are both softened by adamantylidene addition, irrespective of the isomeric structure of Dy₂ScN@C₈₀(Ad). For Dy₃N@C₈₀(Ad), Ad addition substantially increased Dy···Dy coupling constants and the energy spread of exchange-coupled states in comparison to that of Dy₃N@C₈₀ and lifted geometric frustration. As a result, both Dy₃N@C₈₀(Ad) isomers exhibit open hysteresis without pronounced QTM signatures and have a higher blocking temperature of magnetization than the pristine Dy₃N@C₈₀. Our work demonstrates that chemical derivatization can have profound influence on the metal–metal coupling and relaxation of magnetization in metallofullerene molecular magnets.