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.
The other fun tidbit is on heavy reservations you pay per-hour even if you're not using the machine. That's not the case for the light or medium. So keep that in mind when planning out your reservations.
When we switched off EC2 it was financially advantageous to basically give away our heavy instances on the marketplace just to shed the liability.
After this latest round of price cuts, I'm starting to question whether I even want to reserve instances on the lower end. An m3.medium is about $52/month on demand vs $35/month (amortized over 12 months with a 1-year heavy usage reservation).
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.
Maybe my math is off, but I calculated $35/mo for medium utilization m3.medium vs $32/mo for heavy utilization. In that case, I don't think a heavy utilization reservation makes sense but a medium utilization reservation still might (as you're saving ~$20/mo over on-demand).
I tried to give away a heavy instance in the past but to sell on the Reserved Instance marketplace you must have a US bank account. Worth noting if you are considering heavy instances and are based outside the US. You will be stuck with them for the life of the reservation term.
