Q: If I opt in to longer IDs and then opt back out during the transition window, what will happen to resources that were created with longer IDs? Once a resource has been assigned an ID it will not change, so resources that are created with longer IDs will retain the longer IDs regardless of later actions. If you opt in to the longer format, create resources, and then opt out, you will see a mix of long and short resource IDs, even after opting out.