It's a nightmare to change the character order, it's like trying to stack magnets with their poles reversed.
I have to say this a fitting description. That's exactly how it feels. Then again, "party order" is only the tip of the iceberg when it comes to issues tied to this control scheme, so it's not like solving this would make it overall particularly likable anyway.
Party control in Baldur's Gate 3 is a complete mess that begs to be addressed. SAY NO TO THE TOILET CHAIN