In addition to showing that a 'Substantial Change in Circumstances' has occurred, you'll also need to show that your ex is truly unable to care for the child, and would not be able to respond appropriately in case of an emergency. You would have a better chance of this if your ex requires a full-time assistant to cope with daily living tasks (cleaning, cooking, taking care of personal hygiene, etc) or is in the hospital and not expected to return. If other people (neighbors, daycare, family members) are called upon to help care for the child, this is also evidence that your ex cannot parent the child effectively on their own.
Be aware that even if your ex dies, this does not guarantee that you will automatically get custody. For more information on this, read If A Custodial Or Non-Custodial Parent Dies.