  Astrometric positions of radio-emitting active galactic nuclei (AGNs) can be
determined with sub-milliarcsec accuracy using very long baseline
interferometry (VLBI). The usually small apparent proper motion of distant
extragalactic targets allow us to realize the fundamental celestial reference
frame with VLBI observations. However, long-term astrometric monitoring may
reveal extreme changes in some AGN positions. Using new VLBI observations in
2018-2021, we show here that four extragalactic radio sources (3C48, CTA21,
1144+352, 1328+254) have a dramatic shift in their positions by 20-130
milliarcsec over two decades. For all four sources, the apparent positional
shift is caused by their radio structure change.
