  We calculate the energy release associated with a strong first-order phase
transition, from normal phase N to an "exotic" superdense phase S, in a
rotating neutron star. Such a phase transition, accompanied by a density jump
rho_N --> rho_S, is characterized by rho_S/rho_N > 3/2(1+P_0/rho_N c^2), where
P_0 is the pressure, at which phase transition occurs. Configurations with
small S-phase cores are then unstable and collapse into stars with large
S-phase cores. The energy release is equal to the difference in mass-energies
between the initial (normal) configuration and the final configuration
containing an S-phase core, total stellar baryon mass and angular momentum
being kept constant. The calculations of the energy release are based on
precise numerical 2-D calculations. Polytropic equations of state (EOSs) as
well as realistic EOS with strong first-order phase transition due to kaon
condensation are used. For polytropic EOSs, a large parameter space is studied.
For a fixed "overpressure", dP, defined as the relative excess of central
pressure of collapsing metastable star over the pressure of equilibrium
first-order phase transition, the energy release E_rel does not depend on the
stellar angular momentum. It coincides with that for nonrotating stars with the
same dP. Therefore, results of 1-D calculations of E_rel(dP) for non-rotating
stars can be used to predict, with very high precision, the outcome of much
harder to perform 2-D calculations for rotating stars with the same dP. This
result holds also for dP_min < dP < 0, corresponding to phase transitions with
climbing over the energy barrier separating metastable N-phase configurations
from those with an S-phase core. Such phase transitions could be realized in
the cores of newly born, hot, pulsating neutron stars.
