Vaughn has an older iPhone 4S, and is wondering if he should upgrade that to iOS 9. He had some issues upgrading from iOS 7 to 8, though. Leo says that Apple won't allow an update to even go through if it isn't going to work on the device. However, he still may run into some issues. His phone may be too small, with only 8GB of storage. It may slow some things down too.
Vaughn can read more about the risks of upgrading at Gizmodo.com