There are a lot of people who always run balances, so interest is always accruing. It won't change a whole lot for them in that regard.

There's probably people on the margin will will pay late, then be hit with interest. Or they'll be late and cancel their no-interest grace period.

Banks are complaining about this, but I think in the net their income won't change all that much. They'll probably also rates rates slightly, and/or make penalty interest rates more common, or stricter.