The “change” banner pops up on all reservations during a specific window, usually about a 4-6 week period, during which Southwest is making some adjustments in the schedule. The great majority of flights are not affected. Even if your flight is not impacted by any schedule change, but the banner appears for your itinerary, Southwest allows a one-time change within the specified parameters; consider it a gift, as there is no change in fare, even if the new flight/date would be more expensive, at the time of the change.
You do not need to do anything, other than double-check for any changes that may have been made by Southwest. Sometimes, Southwest sends an email about a change to your trip; but, some have indicated they never received notification.
For quite a while, the “change” banner has been popping up, every week or two, as schedule adjustments are made. As bookings improve and business stabilizes, I anticipate the banner to eventually disappear.