I can't speak for Basic Economy but I treat major schedule changes like a golden ticket. In my experience I've been able to go back at any time, even months later for changes on tickets booked well in advance, and been able to make any reasonable change to the same destination, even changing to flights a day earlier. I don't know how much it helps that I'm diamond, but I've never had pushback.
Even on other airlines where I have no status, I've called to make a change to a different flight the same day over a month after the schedule change happened, and I haven't had any problems.