You can call describe_valid_db_instance_modifications to learn what modifications you can make to your DB instance. You can use this information when you call modify_db_instance.
See https://paws-r.github.io/docs/rds/describe_valid_db_instance_modifications.html for full documentation.
rds_describe_valid_db_instance_modifications(DBInstanceIdentifier)[required] The customer identifier or the ARN of your DB instance.