I tagged post #3512507 with long hair before, but I believe it is very long hair.

I added very long hair to the post but it doesn't let me remove the old one (long hair). Why does this happen and how do I remove it?

Very long hair implicates long hair.
So if you add vlh you also have lh in this post.

You can see this in the very long hair wiki.

Basically, very long hair is long, so it also falls under the long hair umbrella.

Conversely, not all long hair is very long.

In the same vein, very short hair implies short hair because of similar logic.

You can check at the bottom of the wiki page for the "This tag implicates long_hair (learn more)." text that implies it automatically adds another tag.

Thanks you all, makes sense!