  Over the last decade, tremendous strides have been achieved in our
understanding of magnetism in main sequence hot stars. In particular, the
statistical occurrence of their surface magnetism has been established (~10%)
and the field origin is now understood to be fossil. However, fundamental
questions remain: how do these fossil fields evolve during the post-main
sequence phases, and how do they influence the evolution of hot stars from the
main sequence to their ultimate demise? Filling the void of known magnetic
evolved hot (OBA) stars, studying the evolution of their fossil magnetic fields
along stellar evolution, and understanding the impact of these fields on the
angular momentum, rotation, mass loss, and evolution of the star itself, is
crucial to answering these questions, with far reaching consequences, in
particular for the properties of the precursors of supernovae explosions and
stellar remnants. In the framework of the BRITE spectropolarimetric survey and
LIFE project, we have discovered the first few magnetic hot supergiants. Their
longitudinal surface magnetic field is very weak but their configuration
resembles those of main sequence hot stars. We present these first
observational results and propose to interpret them at first order in the
context of magnetic flux conservation as the radius of the star expands with
evolution. We then also consider the possible impact of stellar structure
changes along evolution.
