Reserved instances

Reserved Instances (RIs) have a normalization factor [1], such that 1 x t3.medium is equal to 0.5 x t3.large. RIs are applied by the billing system and will take the instance family into account when applying. Therefore, you don't need to do anything on the RI side to change from a t3.medium to a t3.large. If you are asking about how to migrate from a t3.medium instance to a t3.large instance without downtime on the EC2/compute front, then that is a different question and the RI is not involved.

[1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html

Thank you very much