Multi-epoch spectroscopy of Mg II broad absorption line transitions
  Built upon a sample of 134 quasars that was dedicated to a systematic study
of \mgii-BAL variability from Yi et al. (2019a), we investigate these quasars
showing \mgii-BAL disappearance or emergence with the aid of at least three
epoch optical spectra sampled more than 15 yr in the observed frame. We
identified 3/3 quasars undergoing pristine/tentative BAL transformations. The
incidence of pristine BAL transformations in the sample is therefore derived to
be 2.2$_{-1.2}^{+2.2}$\%, consistent with that of high-ionization BAL
transformations from the literature. Adopting an average \mgii-BAL
disappearance timescale of rest-frame 6.89 yr among the six quasars, the
average characteristic lifetime of \mgii\ BALs in the sample is constrained to
be $>$160 yr along our line of sight. There is a diversity of BAL-profile
variability observed in the six quasars, probably reflecting a variety of
mechanisms at work. Our investigations of \mgii-BAL transitions, combined with
observational studies of BAL transitions from the literature, imply an overall
FeLoBAL/LoBAL$\rightarrow$HiBAL/non-BAL transformation sequence along with a
decrease in reddening. This sequence is consistent with the evacuation models
for the origin of commonly seen blue quasars, in which LoBAL quasars are in a
shorted-lived, blowout phase.
