  Using the density matrix equations (DME) for high scale leptogenesis based on
the type I seesaw mechanism, in which the CP violation (CPV) is provided by the
low-energy Dirac or/and Majorana phases of the neutrino mixing (PMNS) matrix,
we investigate the 1-to-2 and the 2-to-3 flavour regime transitions, where the
1, 2 and 3 leptogenesis flavour regimes in the generation of the baryon
asymmetry of the Universe $\eta_B$ are described by the Boltzmann equations.
Concentrating on the 1-to-2 flavour transition we determine the general
conditions under which $\eta_B$ goes through zero and changes sign in the
transition. Analysing in detail the behaviour of $\eta_B$ in the transition in
the case of two heavy Majorana neutrinos $N_{1,2}$ with hierarchical masses,
$M_1 \ll M_2$, we find, in particular, that i) the Boltzmann equations in many
cases fail to describe correctly the generation of $\eta_B$ in the 1, 2 and 3
flavour regimes, ii) the 2-flavour regime can persist above (below) $\sim
10^{12}$ GeV ($\sim 10^9$ GeV), iii) the flavour effects in leptogenesis
persist beyond the typically considered maximal for these effects leptogenesis
scale of $10^{12}$ GeV. We further determine the minimal scale
$M_{1\text{min}}$ at which we can have successful leptogenesis when the CPV is
provided only by the Dirac or Majorana phases of the PMNS matrix as well as the
ranges of scales and values of the phases for having successful leptogenesis.
We show, in particular, that when the CPV is due to the Dirac phase $\delta$,
there is a direct relation between the sign of $\sin \delta$ and the sign of
$\eta_B$ in the regions of viable leptogenesis in the case of normal
hierarchical light neutrino mass spectrum; for the inverted hierarchical
spectrum the same result holds for $M_1 \lesssim 10^{13}$ GeV. The considered
scenarios of leptogenesis are testable and falsifiable in low-energy neutrino
experiments.
