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Mixed anion compounds afford considerable compositional diversity and tunability of function. A challenging aspect of anion control of properties of periodic solids is to preserve the crystal lattice while substituting for different anions of widely varying size and hardness. Post-synthetic modification routes that place cations or anions in non-equilibrium configurations represent a promising means of decoupling composition and crystal structure and for accessing metastable solids; however, such methods remain relatively unexplored for anion placement. Here, we report the synthesis of LaOI nanocrystals by a non-hydrolytic sol-gel condensation reaction and their transformation into LaOBr, LaOCl, and LaOF nanocrystals along hard-soft-acid-base principles using post-synthetic metathesis reactions with ammonium halides. Anion displacement proceeds along halide planes preserving the tetragonal matlockite structure. Energy-variant X-ray excited optical luminesce signatures of alloyed Tb3+-ions serves as a sensitive quantum reporter of the preservation of the cation sublattice and hardening of the crystal structure upon anion replacement.