I think your in luck. Looks like your existing reserved instances will see the savings as well.
"This customer was in the process of switching to 3-year Heavy Utilization Reserved Instances (seeing most of their instances are running steady state) for a whopping savings of 55%. Now, with the new EC2 price drop we're announcing today, this customer will save another 37% on these Reserved Instances."
I think that was carefully worded - note the "in the process of switching". The email sent to existing owners this morning says the new pricing will only apply to reserved instances purchased since March 6.
"This customer was in the process of switching to 3-year Heavy Utilization Reserved Instances (seeing most of their instances are running steady state) for a whopping savings of 55%. Now, with the new EC2 price drop we're announcing today, this customer will save another 37% on these Reserved Instances."