  Recent monitoring campaigns designed to map the accretion regime in AGN show
major discrepancies with models where the optical/ultraviolet (UV) is produced
by X-ray-illuminated, optically thick disc material within a few hundred
gravitational radii. However, these campaigns only monitored X-rays below $10$
keV, whereas the bolometric luminosity for most of these AGN peaks above $50$
keV. We use data from the recent multiwavelength campaign by \cite{E17} on NGC
4151 - the only AGN bright enough to be monitored at higher energies with
\textit{Swift} BAT. We develop a spectral-timing model with a hot corona, warm
Comptonisation, and outer standard disc. This fits the time-averaged spectrum
well, but completely fails to match the UV variability predicted from the X-ray
lightcurve. However, it reveals that NGC 4151 had a bolometric luminosity
around $1.4\%$ of the Eddington luminosity during this campaign, close to the
luminosity at which AGN show a `changing-state' transition, where the broad
optical lines disappear. Stellar mass black holes show a state transition at a
similarly low Eddington fraction, which is broadly interpreted as the inner
disc being replaced by an optically thin flow. We find that the UV lightcurve
can instead be matched by reprocessing of the X-ray flux on size scales of the
broad line region (BLR; $1.5-20$ light-days) and rule out there being optically
thick material inwards of this, as expected if the thin disc is replaced by the
flow below the inner radius of the BLR. These results emphasise the need for
even longer-timescale, multiwavelength monitoring campaigns on variable AGN.

High Energy Astrophysical Phenomena