Cation exchange has emerged as a powerful and versatile synthetic strategy for fabricating colloidal nanocrystals with tunable compositions and structures. While extensively studied in transition metal sulfides and selenides, its application to transition metal phosphides (TMPs) remains largely unexplored. Here, we report the first successful cation exchange synthesis of Co₂P and Ni₂P nanoplatelets (NPLs) using Cu₃P NPLs as templates. The study demonstrates that the hexagonal phosphorus framework of Cu₃P remains largely intact during the transformation to orthorhombic Co₂P, preserving the original NPL morphology. However, the synthesis of hexagonal Ni₂P NPLs leads to significant morphological distortions, highlighting structural differences in anionic frameworks between Cu₃P and Ni₂P. Partial cation exchange further enabled the formation of heterostructured Co₂P-Cu₃P and Ni₂P-Cu₃P NPLs with adjustable compositions. The synthesized Co₂P and Ni₂P NPLs were evaluated as electrocatalysts for the oxygen evolution reaction (OER) in alkaline conditions, with Co₂P demonstrating outstanding performance, achieving an overpotential of 270 mV and a Tafel slope of 60.4 mV·dec⁻¹ at 10 mA·cm⁻², along with remarkable long-term stability. These findings establish cation exchange as a viable strategy for synthesizing TMP nanocrystals with controlled composition and morphology, expanding the design space for next-generation electrocatalysts.