A semicrystalline cross-linked polymer exhibiting two-way conversion between totally different mechanical properties was obtained. Furyl-telechelic poly(1,4-butylene succinate-co-1,3-propylene succinate) prepolymer (PBPSF2) was prepared and polymerized with tris-maleimide linker (M-3) by the Diels-Alder (DA) reaction in the bulk state. The DA reactions with M-3 above and below the melting temperature of PBPSF2 gave network polymers with relatively soft and hard properties, respectively, though the degrees of crystallinity of them were very similar. The large difference in mechanical properties was attributed to the difference in size of crystallites. By combining the two dynamic processes of melt-recrystallization and depolymerization-repolymerization, we can freely convert the PBPSF2 + M-3 system back and forth between hard and soft materials.