  The X-ray source 3XMM~J150052.0+015452 was discovered as a spectacular tidal
disruption event candidate during a prolonged ($>11$ yrs) outburst (Lin et al.
2017). It exhibited unique quasi-soft X-ray spectra of characteristic
temperature $kT\sim0.3$ keV for several years at the peak, but in a recent
Chandra observation (10 yrs into the outburst) a super-soft X-ray spectrum of
$kT\sim0.15$ keV was detected. Such dramatic spectral softening could signal
the transition from the super-Eddington to thermal state or the temporary
presence of a warm absorber. Here we report on our study of four new XMM-Newton
follow-up observations of the source. We found that they all showed super-soft
spectra, suggesting that the source had remained super-soft for $>5$ yrs. Then
its spectral change is best explained as due to the super-Eddington to thermal
spectral state transition. The fits to the thermal state spectra suggested a
smaller absorption toward the source than that obtained in Lin et al. (2017).
This led us to update the modeling of the event as due to the disruption of a
0.75 msun star by a massive black hole of a few$\times10^5$ msun. We also
obtained two HST images in the F606W and F814W filters and found that the dwarf
star-forming host galaxy can be resolved into a dominant disk and a smaller
bulge. No central point source was clearly seen in either filter, ruling out
strong optical emission associated with the X-ray activity.
