If you have Standard RIs, you can only merge them using modification feature if start & expiry dates on both RIs are the same. If, for example, you bought one .nano RI in April, and another .nano RI in July, they will have different start & end dates, so you would not be able to merge them.
If you are using Linux/UNIX RIs with Regional scope and Shared tenancy, you don't need to merge them, because they are size flexible and will be applied to your larger running instance with other matching attributes automatically.
However, if both your RIs are Convertible, you can select both of them, and then use the "Exchange" feature to convert them into one bigger instance.
Hi There If you are running Linux then you don't have to do anything, the two t4g.nano RI's will apply to your t4g.micro instance. Please see "Instance Size Flexibility" in the documentation here
You can also merge two or more Reserved Instances into a single Reserved Instance. For example, if you have four t2.small Reserved Instances of one instance each, you can merge them to create one t2.large Reserved Instance. For more information, see Support for modifying instance sizes.
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