We present the results of Langevin dynamics simulations on a coarse-grained model for a structural transition in crystalline cellulose pertinent to the cellulose degradation problem. We analyze two different cellulose crystalline forms: cellulose Iβ (the natural form of cellulose) and cellulose III(I) (obtained after cellulose Iβ is treated with anhydrous liquid ammonia). Cellulose III(I) has been the focus of wide interest in the field of cellulosic biofuels, as it can be efficiently hydrolyzed to readily fermentable glucose (its enzymatic degradation rates are up to 5-fold higher than those of cellulose Iβ). The coarse-grained model presented in this study is based on a simplified geometry and on an effective potential mimicking the changes in both intracrystalline hydrogen bonds and stacking interactions during the transition from cellulose Iβ to cellulose III(I). The model reproduces both structural and thermomechanical properties of cellulose Iβ and III(I). The work presented herein describes the structural transition from cellulose Iβ to cellulose III(I) as driven by the change in the equilibrium state of two degrees of freedom in the cellulose chains. The structural transition from cellulose Iβ to cellulose III(I) is essentially reduced to a search for optimal spatial arrangement of the cellulose chains.