Your friend who moved is allowed to make new friends. Being replaced and being added to are different things.

Hearing new names from somebody who moved can feel like evidence of being displaced, and it almost never is. People do not have one slot that a new friend takes over. They build a life where they are, and your place in it is not what is being filled. Added to is the usual reality, however it reads from a distance.

When they talk about the new people, ask about them properly instead of going quiet. It is worth saying that you miss them, which is a different sentence from being hurt that they have company. Then book something.