In my past experience with this type of promo over the last couple of years I did same day changes and still qualified. I think the change may eliminate qualification would be for moving a flight past the promotion fly by date as mentioned by @epcot. one time SW did a schedule change I changed my date still within the promotion travel by date and qualified so you should be good for same day changes
Personally it would be a headache for the airline if they eliminated qualification for same-day changes because some have to be changed due to IROPS and it wouldn’t be fair to those customers to say you no longer qualify because we had to rebook you due to a flight delay
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