  The nature of the so-called ``changing-look'' (CL) active galactic nucleus
(AGN), which is characterized by spectral-type transitions within $\sim10$~yr,
remains an open question. As the first in our series of studies, we here
attempt to understand the CL phenomenon from a view of the coevolution of AGNs
and their host galaxies (i.e., if CL-AGNs are at a specific evolutionary stage)
by focusing on the SDSS local ``partially obscured'' AGNs in which the stellar
population of the host galaxy can be easily measured in the integrated spectra.
A spectroscopic follow-up program using the Xinglong 2.16~m, Lick/Shane 3~m,
and Keck 10~m telescopes enables us to identify in total 9 CL-AGNs from a
sample of 59 candidates selected by their mid-infrared variability. Detailed
analysis of these spectra shows that the host galaxies of the CL-AGNs are
biased against young stellar populations and tend to be dominated by
intermediate-age stellar populations. This motivates us to propose that CL-AGNs
are probably particular AGNs at a specific evolutionary stage, such as a
transition stage from ``feast'' to ``famine'' fueling of the supermassive black
hole. In addition, we reinforce the previous claim that CL-AGNs tend to be
biased against both a high Eddington ratio and a high bolometric luminosity,
suggesting that the disk-wind broad-line-region model is a plausible
explanation of the CL phenomenon.
