Best practices when upgrading RDS instance generations?

Some considerations:

Following are some things to consider when scaling up an RDS instance:

• Before you scale, make sure you have the correct licensing in place for commercial engines (SQL Server, Oracle) especially if you Bring Your Own License (BYOL). One important thing to call out is that for commercial engines, you are restricted by the license, which is usually tied to the CPU sockets or cores.
• Determine when you want to apply the change. You have an option to apply the change immediately or during the maintenance window specified for the instance.
• Storage and instance type are decoupled. When you scale your database instance up or down, your storage size remains the same and is not affected by the change. You can separately modify your DB instance to increase the allocated storage space or improve the performance by changing the storage type (such as to General Purpose SSD to Provisioned IOPS SSD).
• There is minimal downtime when you are scaling up on a Multi-AZ environment because the standby database gets upgraded first, then a failover will occur to the newly sized database. A Single-AZ instance will be unavailable during the scale operation.

Take a look at: https://aws.amazon.com/blogs/database/scaling-your-amazon-rds-instance-vertically-and-horizontally/