Attention: Here be dragons

This is the latest (unstable) version of this documentation, which may document features not available in or compatible with released stable versions of Godot.

Checking the stable version of the documentation...

Changer de scène manuellement

Sometimes it helps to have more control over how you swap scenes around. A Viewport's child nodes will render to the image it generates. This holds true even for nodes outside of the "current" scene. Autoloads fall into this category, and also scenes which you instantiate and add to the tree at runtime:

var simultaneous_scene = preload("res://levels/level2.tscn").instantiate()

func _add_a_scene_manually():
    # This is like autoloading the scene, only
    # it happens after already loading the main scene.
    get_tree().root.add_child(simultaneous_scene)

public Node simultaneousScene;

public MyClass()
{
    simultaneousScene = ResourceLoader.Load<PackedScene>("res://levels/level2.tscn").Instantiate();
}

public void _AddASceneManually()
{
    // This is like autoloading the scene, only
    // it happens after already loading the main scene.
    GetTree().Root.AddChild(simultaneousScene);
}

To complete the cycle and swap out the new scene with the old one, you have a choice to make. Many strategies exist for removing a scene from view of the Viewport. The tradeoffs involve balancing operation speed and memory consumption, as well as balancing data access and integrity.

Delete the existing scene. SceneTree.change_scene_to_file() and SceneTree.change_scene_to_packed() will delete the current scene immediately. You can also delete the main scene. Assuming the root node's name is "Main", you could do get_node("/root/Main").free() to delete the whole scene.
- Décharger la mémoire.
  Avantage : La RAM ne traîne plus de poids mort.
  
  Inconvénient : Le retour à cette scène est maintenant plus coûteux car il faut la recharger en mémoire (cela prend du temps ET de la mémoire). Ce n'est pas un problème si retourner rapidement n'est pas nécessaire.
  
  Inconvénient : Ne plus avoir accès aux données de cette scène. Pas de problème si l'utilisation de ces données n'est pas rapidement nécessaire.
  
  Note : Il peut être utile de préserver les données d'une scène qui sera bientôt supprimée en rattachant un ou plusieurs de ses nœuds à une autre scène, ou même directement au SceneTree.
- Arrêts de traitement.
  Pro: No nodes means no processing, physics processing, or input handling. The CPU is available to work on the new scene's contents.
  
  Inconvénient : le traitement et la gestion des entrées de ces nœuds ne fonctionnent plus. Pas de problème si l'utilisation des données mises à jour n'est pas nécessaire.
Hide the existing scene. By changing the visibility or collision detection of the nodes, you can hide the entire node sub-tree from the player's perspective.
- La mémoire existe toujours.
  Pro: You can still access the data if needed.
  
  Avantage : Il n'est pas nécessaire de déplacer des nœuds pour enregistrer les données.
  
  Con: More data is being kept in memory, which will be become a problem on memory-sensitive platforms like web or mobile.
- Le traitement se poursuit.
  Pro: Data continues to receive processing updates, so the scene will keep any data within it that relies on delta time or frame data updated.
  
  Avantages : Les nœuds sont toujours membres des groupes (puisque les groupes appartiennent au SceneTree).
  
  Con: The CPU's attention is now divided between both scenes. Too much load could result in low frame rates. You should be sure to test performance as you go to ensure the target platform can support the load from this approach.
Remove the existing scene from the tree. Assign a variable to the existing scene's root node. Then use Node.remove_child(Node) to detach the entire scene from the tree.
- Memory still exists (similar pros/cons as hiding it from view).
- Processing stops (similar pros/cons as deleting it completely).
- Pro: This variation of "hiding" it is much easier to show/hide. Rather than potentially keeping track of multiple changes to the scene, you only need to call the add/remove_child methods. This is similar to disabling game objects in other engines.
- Inconvénient : Contrairement au masquage de la vue uniquement, les données contenues dans la scène deviendront obsolètes si elles dépendent du temps delta, des entrées, des groupes ou d'autres données dérivées de l'accès à SceneTree.

There are also cases where you may wish to have many scenes present at the same time, such as adding your own singleton at runtime, or preserving a scene's data between scene changes (adding the scene to the root node).

get_tree().root.add_child(scene)

GetTree().Root.AddChild(scene);

Another case may be displaying multiple scenes at the same time using SubViewportContainers. This is optimal for rendering different content in different parts of the screen (e.g. minimaps, split-screen multiplayer).

Each option will have cases where it is best appropriate, so you must examine the effects of each approach, and determine what path best fits your unique situation.