  We report high field de Haas-van Alphen (dHvA) effect measurements in
CeRh$_2$Si$_2$ both below and above the first-order 26 T metamagnetic
transition from an antiferromagnetic to a polarized paramagnetic state. The
dHvA frequencies observed above the transition are much higher than those
observed below, implying a drastic change of the Fermi-surface size. The dHvA
frequencies above the transition and their angular dependence are in good
agreement with band-structure calculations for LaRh$_2$Si$_2$, which correspond
to CeRh$_2$Si$_2$ with localized $f$ electrons. Given that the $f$ electrons
are also localized at low fields in CeRh$_2$Si$_2$, the Fermi-surface
reconstruction is due to the suppression of antiferromagnetism and the
restoration of the crystallographic Brillouin zone rather than the
delocalization of the $f$ electrons. This example suggests that the intuitive
notation of "small" and "large" Fermi surfaces commonly used for localized and
itinerant $f$ electrons, respectively, requires careful consideration, taking
into account the modification of the Brillouin zone in the antiferromagnetic
state, when interpreting experimental results.
