  The evolution of central stars of planetary nebulae was so far documented in
just a few cases. However, spectra collected a few decades ago may provide a
good reference for studying the evolution of central stars using the emission
line fluxes of their nebulae. We investigated evolutionary changes of the
[OIII] 5007 A line flux in the spectra of planetary nebulae. We compared
nebular fluxes collected during a decade or longer. We used literature data and
newly obtained spectra. A grid of Cloudy models was computed using existing
evolutionary models, and the models were compared with the observations. An
increase of the [OIII] 5007 A line flux is frequently observed in young
planetary nebulae hosting H-rich central stars. The increasing nebular
excitation is the response to the increasing temperature and hardening
radiation of the central stars. We did not observe any changes in the nebular
fluxes in the planetary nebulae hosting late-type Wolf-Rayet (WR) central
stars. This may indicate a slower temperature evolution (which may stem from a
different evolutionary status) of late-[WR] stars. In young planetary nebulae
with H-rich central stars, the evolution can be followed using optical spectra
collected during a decade or longer. The observed evolution of H-rich central
stars is consistent with the predictions of the evolutionary models provided in
the literature. Late-[WR] stars possibly follow a different evolutionary path.
