  In this paper we present an analysis of the first high-resolution full Stokes
vector spectropolarimetric observations of the active M dwarf AD Leo. Based on
observations collected in 2016 with the ESPaDOnS instrument at CFHT, we derived
the least-squares deconvolved Stokes profiles and detected linear polarisation
signatures in spectral lines. At the same time, we discovered that the circular
polarisation profiles corresponding to our data set are significantly weaker
compared to all archival spectra of AD Leo, which exhibited approximately
constant profiles over the timescale of at least 6 years until 2012. Magnetic
maps obtained using Zeeman Doppler imaging confirm the sudden change in the
surface magnetic field. Although the total magnetic field energy decreased by
about 20% between 2012 and 2016, the field component responsible for the
observed circular polarisation signatures corresponds to a stronger field
occupying a smaller fraction of the stellar surface in the more recent map.
These results represent the first evidence that active M dwarfs with
dipole-dominated axisymmetric field topologies can undergo a long-term global
magnetic variation.
