The mainstream professional polls have been reliable for some time. They typically get within 2-3pt margin of error when taken a few days before the election, and they serve as a useful bellweather to track public sentiment. In this case, it appears jettisoning Biden is bringing voters back to the Dem fold. That's what Dems were banking on, and I'd consider it a good thing given how much I don't want Trump to be President.
Vote like they don’t exist.
People who obsessively follow polling are the ones most likely to vote.
Stepping aside may honestly be the single greatest show of leadership Biden has done in his lifetime. If the polls are swinging, it's as much due to him making such a historical and respectable move as it is people surprised and refreshed by Kamala Harris.