This is such a hugely loaded an impossible question. XD You’re basically asking me what the most abhorrent writing choice in media has ever been. And I love it.

I choose spite and maximum reach.

Every the main characters/villains in the Harry Potter franchise are trans now.

Although if I were going for a change that would effect me personally instead of the change that would do the most good in the world, I would change the ending of the awful movie AI: Artificial Intelligence. I would let him have his mom.