  The beginning of photoionization marks the transition between the
post-Asymptotic Giant Branch (post-AGB) and planetary nebula (PN) phases of
stars with masses < 8 M_sun. This critical phase is difficult to observe, as it
lasts only a few decades. The combination of jets and magnetic fields, the key
agents of PNe shaping, could give rise to synchrotron emission, but this has
never been observed before in any PNe, since free-free emission from the
ionized gas is expected to dominate its radio spectrum. In this paper we report
radio continuum observations taken with the Australia Telescope Compact Array
between 1 and 46 GHz of the young PN IRAS 15103-5754. Our observations in
2010-2011 show non-thermal emission compatible with synchrotron emission from
electrons accelerated at a shock with spectral index $\alpha \simeq -0.54$.
However, in 2012, the spectral index $\alpha \simeq -0.28$ is no longer
compatible with synchrotron emission in these types of processes. Several
hypothesis are discussed to explain this change. The more plausible ones are
related to the presence of the newly photoionized region in this young PN:
either energy loss of electrons due to Coulomb collisions with the plasma, or
selective suppression of synchrotron radiation due to the Razin effect. We
postulate that the observed flattening of non-thermal radio spectra could be a
hallmark identifying the beginning of the PN phase.
