The M9 estimate comes from Harvard using the 300+ s period surface waves. So if there were even slower release of moment, they should have seen a trend of increasing moment release as they examined even longer period waves, and I've heard no such report. More telling, most large earthquakes do not have much moment release after the end of the seismically measured rupture time.

There was an event in Japan a couple of years ago whose moment estimate doubled when derived from geodetic data, but I think the timescale was weeks, not hours. Also, 1960 Chile has been alleged to have an M9.5 PRECURSOR, but I'm not sure I believe it.

This is the biggest earthquake in a while so we may see something new, but I'd guess it is unlikely.