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Built upon a sample of 134 quasars that was dedicated to a systematic studyof \mgii-BAL variability from Yi et al. (2019a), we investigate these quasarsshowing \mgii-BAL disappearance or emergence with the aid of at least threeepoch optical spectra sampled more than 15 yr in the observed frame. Weidentified 3/3 quasars undergoing pristine/tentative BAL transformations. Theincidence of pristine BAL transformations in the sample is therefore derived tobe 2.2$_{-1.2}^{+2.2}$\%, consistent with that of high-ionization BALtransformations from the literature. Adopting an average \mgii-BALdisappearance timescale of rest-frame 6.89 yr among the six quasars, theaverage characteristic lifetime of \mgii\ BALs in the sample is constrained tobe $>$160 yr along our line of sight. There is a diversity of BAL-profilevariability observed in the six quasars, probably reflecting a variety ofmechanisms at work. Our investigations of \mgii-BAL transitions, combined withobservational studies of BAL transitions from the literature, imply an overallFeLoBAL/LoBAL$\rightarrow$HiBAL/non-BAL transformation sequence along with adecrease in reddening. This sequence is consistent with the evacuation modelsfor the origin of commonly seen blue quasars, in which LoBAL quasars are in ashorted-lived, blowout phase.

Multi-epoch spectroscopy of Mg II broad absorption line transitions