What happens if I decide to terminate an instance for which I paid upfront?

I am trying to understand Amazon EC2 reserved instances pricing structure. It is my understanding that the Reserved Instances are no more than a different pricing for my instances.

My question is what happens if I pay upfront for an instance and later for whatever reason I need to terminate it before all of the period of the instance is completed? Can I use the remaining money paid on another instance or that is gone?

Any clarification will be appreciated,

Thanks!