Currently, renaming .qml files inside the Project pane automatically changes the name across CMake files. This is a very convenient feature.

To make it even more convenient, Qt Creator should rename it also in .qml files. (I think it's safe to run the rename to all the .qml files inside the project folder, even for the ones which are not registered to the project with qt_add_qml_module(), etc. because if they are skipped, it could cause trouble when they are registered back to the project again in the future.)
For example, if a user changed the name of a file MyButton.qml to MyCoolButton.qml, and if it's used in multiple files like this;

// SomeQML.qml
MyButton {
  //...
}

This should also be renamed.

// SomeQML.qml
MyCoolButton {
  // ...
}