  The extragalactic $\gamma$-rays sky observed by Fermi-Large Area Telescope
(LAT) is dominated by blazars. In the fourth release of the Fermi-LAT Point
Source Catalog (4FGL), are sources showing a multifrequency behavior similar to
that of blazars but lacking an optical spectroscopic confirmation of their
nature known as Blazar Candidate of Uncertain type (BCUs). We aim at confirming
the blazar nature of BCUs and test if new optical spectroscopic observations
can reveal spectral features, allowing us to get a redshift estimate for known
BL Lac objects. We also aim to search for and discover changing-look blazars
(i.e., blazars that show a different classification at different epochs). We
carried out an extensive search for optical spectra available in the Large Sky
Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) Data Release 5 (DR5)
archive. We selected sources out of the 4FGL catalog, the list of targets from
our follow-up spectroscopic campaign of unidentified or unassociated
$\gamma$-ray sources, and the multifrequency catalog of blazars: the
Roma-BZCAT. We selected a total of 392 spectra. We also compare some of the
LAMOST spectra with those available in the literature. We classified 20 BCUs
confirming their blazar-like nature. Then we obtained 15 new redshift estimates
for known blazars. We discovered 26 transitional (i.e., changing-look) blazars
that changed their classification. Finally, we were able to confirm the
blazar-like nature of six BL Lac candidates. All remaining sources analyzed
agree with previous classifications. BL Lac objects are certainly the most
elusive type of blazars in the $\gamma$-ray extragalactic sky.
