X-ray spectral variability analyses of Active Galactic Nuclei (AGN) with moderate luminosities and redshifts typically shows a behaviour. Such trend has not yet been investigated for high-luminosity AGNs, nor for a wider redshift range. We present an analysis of the spectral variability based on a large sample of quasars, measured at several different epochs, extracted from the latest release of the XMM-Newton Serendipitous Source Catalogue. Our analysis confirms a softer when brighter trend also for our sample, extending to high luminosity and redshift the general trend previously found. This results will be discussed in relation to current spectral models, such as intrinsic variations of the X-ray primary radiation, or superposition with a constant reflection component.