No, the customer will not have the flexibility to change instance generations. "Region, DB Engine, DB Instance Class, Deployment Type and term length must be chosen at purchase, and cannot be changed later" as indicated on this page.
If a customer would like to purchased T3 reserved instances for RDS, they will have to commit to the T3 family. With Aurora MySQL they have size flexibility -- so they can purchase a T3.Large, and run 4 T3.Smalls to get the discount (or whatever the size factor is). However, they must commit to the T3 family. They will not be able to get the discount by running a T2 RDS instance. That T2 will be billed on-demand rates.
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