I've seen where the console would not populate the dropdown. Can you switch to another browser and see if the behavior is the same? Also try to change with the CLI, see: modify-db-instance
Usually, vertical scaling should be straightforward, however, if you are selecting some of the new RDS instance types. You might have to check the upgrade path, it is usually the combination of engine version support to instance class type. That is just one aspect there could be more on how to troubleshoot to solve the above problem. Can you update your question with from which Instance class type to which Instance class type you are trying to upgrade along with current engine version
Finally, it was a GUI/browser thing. Today everything looks fine. Kinda weird.
I was actually going from r5.4xlarge to a T3.medium.. to save money :)
Thanks for your help !
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