  Transient ultraluminous X-ray sources (ULXs) provide an important link
bridging transient low-mass X-ray binaries and ULXs. Here we report the first
discovery of both a canonical sub-Eddington outburst and an ultraluminous
super-Eddington outburst from an unusual transient ULX, NGC 7793 P9 with a
variability factor higher than $10^3$. Its X-ray spectrum switches between the
typical high/soft state and the steep power-law state during the canonical
outburst. The inner radius of the accretion disk and the disk
temperature--luminosity correlation suggest that P9 harbors a stellar-mass
black hole (BH). At the beginning of the ultraluminous outburst, we observe a
cool outer disk with a hard Comptonized spectrum, implying a transition to the
ULX regime. When the luminosity increases to $L\gtrsim3\times10^{39}$ erg/s, P9
shows a significantly curved spectrum that can be described by either a slim
disk or a strongly curved Comptonized corona. The phenomenological model
suggests that the hot disk observed near the peak of the ultraluminous outburst
is coincidentally consistent with the extension of the thermal track. Utilizing
more physical Comptonized disk models, we suggest that the corona cools down
and the apparent disk-like spectrum is dominated by soft Comptonization. The
significant variability above 1 keV supports this two-component scenario. The
spectral evolution can also be interpreted with the supercritical accretion
model. All these indicate that a canonical black hole X-ray binary can show
properties of a ULX.
