​We report two novel three-dimensional porous coordination polymers (PCPs) of formulas Li₄{Mn₄[Cu₂(Me₃mpba)₂]₃}·68H₂O (2) and K₄{Mn₄[Cu₂(Me₃mpba)₂]₃}·69H₂O (3) obtained—via alkali cation exchange in a single-crystal to single-crystal process—from the earlier reported anionic manganese(II)–copper(II) PCP of formula Na₄{Mn₄[Cu₂(Me₃mpba)₂]₃}·60H₂O (1) [Me₃mpba^4– = N,N′-2,4,6-trimethyl-1,3-phenylenebis(oxamate)]. This postsynthetic process succeeds where the direct synthesis in solution from the corresponding building blocks fails and affords significantly more robust PCPs with enhanced magnetic properties [long-range 3D magnetic ordering temperatures for the dehydrated phases (1′–3′) of 2.0 (1′), 12.0 (2′), and 20.0 K (3′)]. Changes in the adsorptive properties upon postsynthetic exchange suggest that the nature, electrostatic properties, mobility, and location of the cations within the framework are crucial for the enhanced structural stability. Overall, these results further confirm the potential of postsynthetic methods (including cation exchange) to obtain PCPs with novel or enhanced physical properties while maintaining unaltered their open-framework structures.