From Southwest.com EBCI FAQs:
https://www.southwest.com/help/booki...lybird-checkin
If I have EarlyBird Check-In and choose to change my flight, will the EarlyBird Check-In transfer as well?
EarlyBird Check-In can be transferred only if the new flight departs more than 25 hours from the time of the change. If the new flight departs in less than 25 hours, then you will lose your non-refundable EarlyBird Check-In purchase. Additionally, if the change to your reservation involves a new city pair(s), you will need to contact us to have the EarlyBird Check-In transferred.