Although the movement of the mass center of the monomer is restricted by the crystal during the polymerization, the slight change of the bond length before and after the reaction give rise to the shifting of lattice parameters. Consider a real-life topochemical polymerization initiated by irradiation: if monomer beneath the surface polymerizes later due to the light absorption near the surface, the already polymerized layer will shrink or expand, causing unbalanced stress within the crystal. The crystal might break or even lose crystallinity if the stress isn't handled properly.
Using elastic interaction such as weak hydrogen bonds is a common strategy to release the stress. It has been found that the bond length of the hydrogen bond in the crystal would change after polymerization, acting as cushion. Another possible routine is to introduce "soft" parts (C-C or C-O bond free to rotate instead of rigid conjugated system) in the monomer molecule. But it will in turn increase the difficulty of crystallization.