We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO_3-xH_x (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO_3-xH_x, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N^3–/H^–-exchanged product EuTi⁴⁺O_2.82N_0.12□_0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N^3–/O^2– exchange, yielding a fully oxidized Eu³⁺Ti⁴⁺O₂N with the GdFeO₃-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO₃ proceeded through a 1:1 exchange of N^3– with O^2– only above 600 °C and resulted in incomplete nitridation to EuTiO_2.25N_0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO_2.82H_0.18 play a crucial role in promoting anion (N^3–/O^2–) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.