From the FAQ:
Q: What changes to a reservation would disqualify a flight from qualifying for promotional Companion Pass through this offer?
A: Any change to a reservation that falls outside of the promotional requirements (enrollment date, booking date, travel date, etc.) will disqualify the flight from this promotion. In other words, flights must be purchased and/or changed during the promotional period and the new flight(s) must be flown between April 3-May 24, 2023 to meet the terms and conditions of this promotion.
The real question is whether or not Southwest IT is smart enough to disqualify you or not. In the previous promotion, SDC did not disqualify which makes me think you can get away with other changes.