After you have edited a pipeline you have the option to update items in the Planning section and backlog with these changes.
You can update all existing items, selected items or only apply the changes to new items.
If you do save your changes and not update items they will have a hyperlink "Old version" to the right of the pipeline name. Clicking this you can either update to the latest version or view the old version of the pipeline:
Likewise, if you delete a pipeline you will have the option to update items:
Leaving items without updating them will result in a "Deleted pipeline" text hyperlink where you can choose to delete pipeline again and update items: