There is usually no single way to tell you exactly which RDS instance type you should move to. The usual way to “recommend” the right type is to look at the Database CloudWatch metrics and understand what part of the DB is actually being used.

If you’ve already identified under-utilized databases, the next question is what they’re under-utilizing. For example, if CPU is always low, that means you can step down to a smaller instance in the same family without slowing down the performance. If memory usage is consistently low and you are not touching swap, that is another sign you can safely downsize.

On the other hand, if memory is tight all the time, moving to a memory-optimized family may be the better option.