  Changing-look (CL) AGN are unique probes of accretion onto supermassive black
holes (SMBHs), especially when simultaneous observations in complementary
wavebands allow investigations into the properties of their accretion flows. We
present the results of a search for CL behaviour in 412 Swift-BAT detected AGN
with multiple epochs of optical spectroscopy from the BAT AGN Spectroscopic
Survey (BASS). 125 of these AGN also have 14-195 keV ultra-hard X-ray
light-curves from Swift-BAT which are contemporaneous with the epochs of
optical spectroscopy. Eight CL events are presented for the first time, where
the appearance or disappearance of broad Balmer line emission leads to a change
in the observed Seyfert type classification. Combining with known events from
the literature, 21 AGN from BASS are now known to display CL behaviour. Nine CL
events have 14-195 keV data available, and five of these CL events can be
associated with significant changes in their 14-195 keV flux from BAT. The
ultra-hard X-ray flux is less affected by obscuration and so these changes in
the 14-195 keV band suggest that the majority of our CL events are not due to
changes in line-of-sight obscuration. We derive a CL rate of 0.7-6.2 per cent
on 10-25 year time-scales, and show that many transitions happen within at most
a few years. Our results motivate further multi-wavelength observations with
higher cadence to better understand the variability physics of accretion onto
SMBHs.
