You're locked into whatever rate your reservation is for. This is why you see rate fluctuations on the marketplace for the same instance type. It's also why a 3 year reservation may not make sense.
Bummer, though. We just reserved a multi-AZ m3.large RDS instance last week. I wish AWS would take up Google's system of graduated price reductions over steady run duration.
When we switched off EC2 it was financially advantageous to basically give away our heavy instances on the marketplace just to shed the liability.
In this example, it becomes a question of whether it's worth saving $130'ish a year for the liability that a 1-year m3.medium heavy util reservation represents. At the lower end, that's not a huge amount of liability, but it may be a case where I just don't bother reserving m3.mediums anymore because it's a wash.