  We utilize medium-resolution JWST/NIRSpec observations of 164 galaxies at
$z=2.0-9.3$ from the Cosmic Evolution Early Release Science (CEERS) survey to
investigate the evolution of the excitation and ionization properties of
galaxies at high redshifts. Our results represent the first statistical
constraints on the evolution of the [OIII]/H$\beta$ vs. [NII]/H$\alpha$,
[SII]/H$\alpha$, and [OI]/H$\alpha$ ``BPT'' diagrams at $z>2.7$, and the first
analysis of the O32 vs. R23 diagram at $z>4$ with a large sample. We divide the
sample into five redshift bins containing 30-40 galaxies each. The subsamples
at $z\sim2.3$, $z\sim3.3$, and $z\sim4.5$ are representative of the
main-sequence star-forming galaxy population at these redshifts, while the
$z\sim5.6$ and $z\sim7.5$ samples are likely biased toward high specific
star-formation rate due to selection effects. Using composite spectra, we find
that each subsample at $z=2.0-6.5$ falls on the same excitation sequence in the
[NII] and [SII] BPT diagrams and the O32-R23 diagram on average, offset from
the sequences followed by $z=0$ HII regions in the same diagrams. The direction
of these offsets are consistent with high-redshift star-forming galaxies
uniformly having harder ionizing spectra than typical local galaxies at fixed
nebular metallicity. The similarity of the average line ratios suggests that
the ionization conditions of the interstellar medium do not strongly evolve
between $z\sim2$ and $z\sim6$. Overall, the rest-optical line ratios suggest
the $z=2.7-9.3$ CEERS/NIRSpec galaxies at log($M_*/M_{\odot})\sim7.5-10$ have
high degrees of ionization and moderately low oxygen abundances
($\sim0.1-0.3~Z_{\odot}$), but are not extremely metal poor ($<0.1~Z_{\odot}$)
even at $z>6.5$.
