When the user uses “Reschedule cards on change”, they can’t determine whether intervals became longer or shorter overall. This can make the user think that their workload is only getting larger and larger. So I suggest showing a message like this:
{number1} cards will be due sooner. {number2} cards will be due later.
or
{number1} intervals became shorter. {number2} intervals became longer.
My wording is kinda awkward, I’m not sure how to word it well. The idea is to count cards for which the new due date < the old due date, as well as cards for which the new due date > the old due date, and display that.