If the person replaces media with a new file, then add that file to the project.
If the person replaces media using a file that already exists in the project, then don't add it to the project.
The less work and thought for the user, the better: adding the file to the project before you can use it is more work than just being able to select a file for replacement.
If the person replaces media with a new file, then add that file to the project.
If the person replaces media using a file that already exists in the project, then don't add it to the project.
The less work and thought for the user, the better: adding the file to the project before you can use it is more work than just being able to select a file for replacement.