Does Existing RI gets applied after resizing the 2-node ds2.xlarge cluster to a 1-node ra3.xlplus cluster

No, the Reserved Instances for Redshift (and other non-EC2 services, for that matter) apply only if the attributes of the RI match the running node types, which is the same type and size. But because you're upgrading to a new generation Redshift nodes (which is a best practice we advise), I suggest to reach out to support - open a billing support case from your account where you have the existing 3yr RI commitment - and ask if there may be options for you to replace the RI with the new one for a new node type.

Since RIs are a commitment for a term, they generally cannot be cancelled or modified. One of the reasons is that generally RI mechanism helps AWS plan the required capacity for customers who commit to use a certain type of resources. If you change the instance/node type or size, the RIs don't get automatically adjusted. But again, as I suggested, please open a support case and ask if there are replacement options considering that you plan to migrate to latest generation nodes (as per AWS best practices).

Hello Raju,

You actually can convert your DS2 RI to RA3 RI, or at least you could, without necessarily contacting the support:

Please check this page to know more: https://docs.aws.amazon.com/redshift/latest/mgmt/working-with-clusters.html#rs-upgrading-to-ra3

Keep in mind that this might be outdated and this "reserved-node upgrade process" might not be available on the console anymore, so you will have to contact the support. Or if you want to resize to a different size than the recommended you will need to contact them too.

Hope this helps you!