  We analyze a Neutron Star Interior Composition Explorer (NICER) observation
of the black hole X-ray binary MAXI J1820+070 during a transition from type-C
to type-B quasi-periodic oscillations (QPOs). We find that below ~2 keV, for
the type-B QPOs the rms amplitude is lower and the magnitude of the phase lags
is larger than for the type-C QPOs. Above that energy, the rms and phase-lag
spectra of the type-B and type-C QPOs are consistent with being the same. We
perform a joint fit of the time-averaged spectra of the source, and the rms and
phase-lag spectra of the QPOs with the time-dependent Comptonization model
vkompth to study the geometry of the corona during the transition. We find that
the data can be well-fitted with a model consisting of a small and a large
corona that are physically connected. The sizes of the small and large coronae
increase gradually during the type-C QPO phase whereas they decrease abruptly
at the transition to type-B QPO. At the same time, the inner radius of the disc
moves inward at the QPO transition. Combined with simultaneous radio
observations showing that discrete jet ejections happen around the time of the
QPO transition, we propose that a corona that expands horizontally during the
type-C QPO phase, from ~10^{4} km (~800 Rg) to ~10^{5} km (~8000 Rg) overlying
the accretion disc, transforms into a vertical jet-like corona extending over
~10^{4} km (~800 Rg) during the type-B QPO phase.
