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We present radio imaging observations of supernova remnant 1987A at 9 GHz,taken with the Australia Telescope Compact Array over 21 years from 1992 to2013. By employing a Fourier modeling technique to fit the visibility data, weshow that the remnant structure has evolved significantly since day 7000(mid-2006): the emission latitude has gradually decreased, such that theoverall geometry has become more similar to a ring structure. Around the sametime, we find a decreasing trend in the east-west asymmetry of the surfaceemissivity. These results could reflect the increasing interaction of theforward shock with material around the circumstellar ring, and the relativeweakening of the interaction with the lower-density material at higherlatitudes. The morphological evolution caused an apparent break in the remnantexpansion measured with a torus model, from a velocity of 4600+150-200 km/sbetween day 4000 and 7000 to 2400+100-200 km/s after day 7000. However, weemphasize that there is no conclusive evidence for a physical slowing of theshock at any given latitude in the expanding remnant, and that a change ofradio morphology alone appears to dominate the evolution. This is supported byour ring-only fits which show a constant expansion of 3890+/-50 km/s withoutdeceleration between days 4000 and 9000. We suggest that once the emissionlatitude no longer decreases, the expansion velocity obtained from the torusmodel should return to the same value as that measured with the ring model.

Evolution of the Radio Remnant of Supernova 1987A: Morphological Changes from Day 7000