Mistakenly Bought AWS RDS Reserved Instance

To obtain the discount you'd have to switch your RDS instance from m4 to m5 (a good idea anyway, newer faster better cheaper), as it only applies to matching instance types.

Firstly, if the decision is made to cancel the reserved instance, will I still be charged for the 2-3 week period? I'm not currently using the instance type nor do I have any instances set up. Is there any way to freeze the charges until a decision is made? Also, if I am charged, will I receive a refund for those weeks?

Ans : If your cancellation is approved, it will be as of the day the request was raised. Receiving refund for the charges already posted will depend on Billing Adjustment approval. Its not guaranteed. If you are cancelling RI within a month ( before a bill is generated), it is easier to handle

Secondly, I was told that the reserved instance discount would be applicable to existing on-demand instances. However, my current instance is db.m4.4xlarge and the reserved instance is db.m5.4xlarge. Is there any possibility of obtaining the discount as mentioned?

Ans : You can raise a request with concierge team to repurpose the RI for new instance type. This will help as they would know that you still plan to use RI but for different instance. Size flexibility is not available for licenses RDS ( Microsoft SQL and Oracle) Refer size flexibility section : https://aws.amazon.com/rds/reserved-instances/