  We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe
covering $\approx 480-850~$days after maximum light and show that the ejecta
undergoes a rapid ionization shift at $\sim 500~$days after explosion. The
prominent [FeIII] emission lines at $\approx 4600~$\r{A} are replaced with the
permitted FeI+FeII blends at $\sim 4400~$\r{A} and $\sim 5400~$\r{A}. The
$\approx 7300~$\r{A} feature, which is produced by [FeII]+[NiII] at $\lesssim
400~$days after explosion, is replaced by broad ($\approx \pm
15\,000~\rm{km}~\rm{s}^{-1}$) symmetric [CaII] emission. Models predict this
ionization transition occurring $\sim 100$ days later than what is observed,
which we attribute to clumping in the ejecta. Finally, we use the nebular-phase
spectra to test several proposed progenitor scenarios for SN2011fe.
Non-detections of H and He exclude nearby non-degenerate companions, [OI]
non-detections disfavor the violent merger of two white dwarfs, and the
symmetric emission-line profiles favor a symmetric explosion.
