The phase change from the amorphous to crystalline state which occurs upon thermal annealing in prototypical Ge2Sb2Te5 and nitrogen-doped Ge2Sb2Te5 phase-change-materials (PCM) thin films is studied by concomitant, complementary and combined in situ and ex situ X-ray diffraction (XRD) and X-ray reflectivity (XRR) techniques. It is demonstrated that combined in situ X-ray scattering techniques allow accurate investigation and clarification of the structural, morphological and mechanical variations occurring in the films upon crystallization. The crystallization process is correlated with volume shrinkage (densification and thickness reduction) and with structural change with a tensile strain build-up. The comparison of Ge2Sb2Te5 and nitrogen-doped Ge2Sb2Te5 reveals a significant slowdown of the crystallization process, induced by the incorporation of nitrogen. However, the mechanisms involved in the phase change are not strongly modified by the incorporation; rather, the crystallization process is inhibited because of the presence of nitrogen. In this way, different stages of the crystallization process can be observed. The combined XRD/XRR analysis gives new insights on the stress components built up in phase-change materials. First, at the early stage of crystallization, a large hydrostatic tensile stress builds up in the PCM thin film. Afterwards, concomitant grain growth, viscous flow, densification and thickness accommodation are observed, which lead to a partial stress relaxation in the PCM films. This combined characterization technique offers a new approach that may further our understanding of the phase change involved.