Pre-comp import updates are completely non-destructive. When an update is needed on a composition of any type (render layer, render pass, or scene), modifications are applied to a new version of the composition (if you turned on the Backup Working Version option in the Pre-Compositing Import Options dialog box or project preferences), which could be a clone of the working version or a new version recreated from scratch.
Moreover, when an update is applied on a clone of the working version, none of the existing nodes in it are deleted (only new content is added) and existing connections are preserved.
Partial updates are also supported. That is, only some cameras or some render layers and passes could be modified during a pre-composition update.
A scene, render layer, or render pass composition is not updated if it doesn't need to be. You can re-export the entire contents of the scene and only the content that needs to be updated will be.Changing the path or the content of a pre-compositing template will not trigger an update as the Composite application does not detect when the template has changed. In this case, you should use the Recreate Working Version update mode in the Pre-Compositing Import Options dialog box or project preferences.
