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 I-beta (the natural form of cellulose) and cellulose IIII (obtained after cellulose I-beta is treated with anhydrous liquid ammonia). Cellulose IIII 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 I-beta). 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 I-beta to cellulose IIII. The model reproduces both structural and thermomechanical properties of cellulose I-beta and IIII. The work presented herein describes the structural transition from cellulose I-beta to cellulose IIII as driven by the change in the equilibrium state of two degrees of freedom in the cellulose chains. The structural transition from cellulose I-beta to cellulose IIII is essentially reduced to a search for optimal spatial arrangement of the cellulose chains.