  We present a comprehensive spectral analysis of the BeXRB GRO J1008-57 over a
luminosity range of three orders of magnitude using NuSTAR, Suzaku and RXTE
data. We find significant evolution of the spectral parameters with luminosity.
In particular the photon index hardens with increasing luminosity at
intermediate luminosities between $10^{36}$ $-$ $10^{37}$ erg s$^{-1}$. This
evolution is stable and repeatedly observed over different outbursts. However,
at the extreme ends of the observed luminosity range, we find that the
correlation breaks down, with a significance level of at least $3.7\sigma$. We
conclude that these changes indicate transitions to different accretion
regimes, which are characterized by different deceleration processes, such as
Coulomb or radiation breaking. We compare our observed luminosity levels of
these transitions to theoretical predications and discuss the variation of
those theoretical luminosity values with fundamental neutron star parameters.
Finally, we present detailed spectroscopy of the unique "triple peaked"
outburst in 2014/15 which does not fit in the general parameter evolution with
luminosity. The pulse profile on the other hand is consistent with what is
expected at this luminosity level, arguing against a change in accretion
geometry. In summary, GRO J1008-57 is an ideal target to study different
accretion regimes due to the well constrained evolution of its broad-band
spectral continuum over several orders of magnitude in luminosity.
