In-situ small-angle X-ray scattering (SAXS) and low-frequency Raman spectroscopy experiments have been performed to study the mobility and motion of polymer chains upon annealing in a temperature range close to but below the melting temperature. Ultrahigh molecular weight polyethylene, UHMW-PE, was taken as the model polymer, which was crystallized from solution. Regularly stacked lamellar crystals could be obtained after drying the solution-crystallized films. SAXS studies revealed that, upon heating above 110 ̊C, the lamellar thickness (long period) increases to twice the initial value. This quantum increase was confirmed by transmission electron microscopy (TEM). By means of in-situ low-frequency Raman spectroscopy it was observed that during heating a shift occurs in the longitudinal acoustic (LA) Raman frequency to lower values, indicative of an increase in the thickness of the crystalline core. The combined experimental observations indicate that thickening occurs via a mutual chain rearrangement between the adjacent lamellae involving a sliding motion along the chain axis to a doubling of the lamellar thickness. A model to explain the quantum increase, i.e., doubling, is proposed.