AWS: Resizing a RESERVED t2.medium to a m3.large. How pricing works?

I want to hire a reserved instance in Amazon EC2 to host an application.

What really drives me the choose of it, is the RAM capacity. I know I will be running this APP for years, but I do not know how hardware requirements will be changing in the future.

So, lets say I hire a t2.medium reserved instance for a year, and in 6 month I have a boom of demand and I realize I have to migrate to a m3.large.

Do I need to give up 6 month of the already paid t2.medium instance ? Or I can just pay the difference for the 6 month between the t2.medium and the m3.large instance ?

I think flexibility is what really make AWS interesting... but I need to be able to scale up at a reasonable cost...