We serendipitously discovered the FeLoBAL SDSS~J1632+4204 showed the dramatic 
change in its spectrum over 3.4 years in the quasar's frame.
SDSS~J1632+4204 changed to a LoBAL from an FeLoBAL when we observed it 
on 2008 February 26 (UT) with Subaru/FOCAS as part of a program to search for 
Balmer-line absorption in FeLoBAL quasars.
The spectrum taken on 2002 April by SDSS show strong Fe~II absorption lines
while Fe~II absorption became much weaker in our 2008 February spectrum.
On the other hand, equivalent widths of Mg~II and He~I absorption remained constant.
The continuum in 2008 became brighter than that in 2002 by factor 3 at 2600 {\AA} 
in quasar's frame.
The color of the continuum in 2008 also became bluer than that in 2002, and match 
the quasar composite spectrum.

This observed change is consistent with increase of ionization level in BAL gas 
due to increase of nuclear radiation. 
The response time of ionization condition to the radiation is 
anti-proportional to electron density.
The upper limit of time scale of variation (3.4 yr in quasar's frame)
gives us lower limit of electron density, $> 10^{4}$ cm$^{-3}$.
The time scale also limits the size of the absorber, $< 1$ pc. 

The Seyfert galaxy NGC~4151 is also known to switch between FeLoBAL and LoBAL.
If these type-switch are general characteristic among FeLoBALs, the LoBAL phase 
will be longer than FeLoBAL phase because LoBALs are four times more common than 
FeLoBALs.
When we will observe FeLoBALs again, 80\% FeLoBALs would change to LoBALs
if our hypothesis that LoBALs sometimes change to FeLoBALs is correct.
Such a large change in Fe~II absorption line makes contrast with
recent results of C~IV absorption line.
The C~IV absorption line does not show large change in strength over 3-5 years.
This may suggest that lower-ionization Fe~II absorbers have
clumpy/filamentary structures with larger volume density,
floating in higher-ionization C~IV absorbers with lower volume density.

arried out through collaborations