I don't think it is a voter shift. Instead it is a shift in the pollsters reporting more realistic numbers so they don't look stupid on election day (week, month, year however that turns out).
The lopsided early polls are push polls designed to steer stupid and uninformed voters around to certain candidates. Their goal is to capture early mail-ins before more realistic numbers come out. Unfortunately some gotta vote for the winner regardless of qualification or issues...vote early, vote uninformed.
A method to tear apart their bogus polling was to examine the sampling to see the percentage of Dems, Repubs, and Indies sampled.
Time and again they were found to massively oversample Dens and undersample Repubs.
Now many of them won’t even reveal their sampling percentages.