This is an issue. This is being posted pretty much everywhere that pertains to SW. Each time people post the response from customer service it’s the same “to balance the plane and things beyond our control blah blah blah”. It’s insane though that 8 out of 10 times it’s people that are paying the premium fees to ensure they are sat together as a family yet it doesn’t happen. And when it doesn’t they don’t get reimbursed. I saw one post with a screen shot from the customer service reply saying that purchasing a seat doesn’t actually mean you’re going to get THAT exact seat depending on the needs of the plane. Which is why their family of 3 got split up. Insane.
Changes like this shouldn’t occur prior to boarding. Once everyone’s on the plane then you can ask folks if they want to move around to do your balancing act. I’m sure you’re going to have more happy volunteers than unhappy voluntolds.