We present an analysis of high-resolution spectropolarimetric observations of the active M dwarf AD Leo in all four Stokes parameters. We derived the least-squares deconvolved Stokes profiles and detected linear polarisation signatures in spectral lines. This is the first report of a detection of Zeeman linear polarisation in the spectral lines of an M dwarf. Simultaneously, we discovered that the circular polarisation profiles corresponding to our data set are significantly weaker and broader than those present in all archival spectra of AD Leo, which exhibited approximately constant profiles over the period 2006–2012. Magnetic maps obtained using Zeeman Doppler imaging confirm a significant - and apparently rather sudden - change in the surface magnetic field. On the one hand, the topology of the magnetic field is mostly unchanged and remains close to an axisymmetric dipole. On the other hand, the total energy of the large-scale magnetic field has decreased by about 20 percent between 2012 and 2016 and the filling factor of this component has decreased by about a factor of two. These results suggest that the field responsible for the observed circular polarisation signatures corresponds to a stronger field component occupying a smaller fraction of the stellar surface in the more recent map. These results represent the first evidence that an active M dwarf with dipole-dominated axisymmetric field topology can undergo a long-term global magnetic field evolution.