We present three new spectra of the nearby Type Ia supernova (SNIa) 2011fe covering ~480-850days after maximum light and show that the ejecta undergoes a rapid ionization shift at ~500days after explosion. The prominent FeIII emission lines at ~4600{AA} are replaced with FeI+FeII blends at ~4400{AA} and ~5400{AA}. The ~7300{AA} feature, which is produced by [FeII]+[NiII] at <~400days after explosion, is replaced by broad (~+/-15000km/s) symmetric [CaII] emission. Models predict this ionization transition occurring ~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 SN 2011fe. Nondetections of H and He exclude nearby nondegenerate companions, [OI] nondetections disfavor the violent merger of two white dwarfs, and the symmetric emission-line profiles favor a symmetric explosion.