This is definitely one of those questions that's hard to give a definitive answer to. It really depends on what you're using the YAML for and whether not having it on all relevant notes would prevent you from accomplishing your goal.

Let's say I have my watched movies in Obsidian. I have basic YAML like IMDB link, genre, and year released. Later, I decide I want to see what directors I have watched the most. In this instance, not having the director YAML on all of my movies is something I would want to fix.

Continuing the above example, I would do two things to make adapting the new structure easier. First, I would update my template for movies to automatically include that field for any new movies I create. Second, I would probably use Dataview to create a utility page to identify movies with no director. The logic of the query would basically be "select all from movies where no director". That way I could systematically update my existing notes without having to look through all of them individually.

Hopefully this gives you a couple of ideas. If you have any follow-up questions, let me know!