When performing a stack update, it can be unclear exactly what changes are going to be made to your stack. Depending on which resource you are changing, you may find that it will need to be deleted and recreated in order to implement your change. This, of course, is completely undesired behavior if the resource in question contains data you'd like to keep. Keep in mind that RDS databases can be a particular pain point.

To mitigate this situation, CloudFormation allows you to create and review a change set prior to executing the update. The change set shows you which operations CloudFormation intends to perform on your resources. If the change set looks good, you can choose to proceed. If you don't like what you see, you can delete the change set and choose another course of action—perhaps choosing to create and switch to an entirely new stack to avoid a service outage.