Even if you come across a situation where your primary key has to change, it seems like you could "simply" migrate your initial primary key to the "additional column" concept referred to in the article, and add your new primary key as needed.

I can't imagine that a primary key migration would be easy even if you'd followed the author's suggestions around having an internal key and an external key, and I feel like if you're optimizing for a future primary key migration you might be focusing on the wrong part of your product.