[How is being able to have a romantic interaction with him, just like every other LI does, rewriting him?
Because the story is already written. Changing it is re-writing it. The amount of times AA already received changes, you’d think larian would’ve added a hug to the epilogue if they thought it was necessary. Clearly they thought changing the facial expressions in kisses was, so they did it. Like I said, Astarion is already fleshed out and well written. Personally I feel AA also got way more attention from devs than UA already, and his romance is great the way it is. Headcanon one way or the other, the canon romantic interactions with AA are already very good.
This is not changing the story, it's adding a mechanic that already exists in the game. We could kiss him just fine up until this point.