The SDSS Spectroscopic surveys have been made by two generations of spectrographs and improvements of software/templates pipelines along the data releases.
Some objects have been targeted multiple times with different outputs due to these improvements, but it's hard to find physical evolution measurements for a non supernova object like a quasar.
I wonder what are the intrinsic causes of object evolution seen left after calibration of the seeing conditions of observations on measurements over a few years.
As an illustration I post 3 superposed plots of quasar SDSS J234614.26+142022.2 with successive redshifts estimations of 3.015211(SDSS legacy), 3.015716(segue2), 3.016687(boss) in 11 years time frame
Spectra are smoothed using the Savitzky-Golay-filter of degree 4 and window size 33 Å for clarity.