You can change floating IP addresses after migrating on-premises databases or other cloud databases to RDS.
After a floating IP address is changed, the domain name needs to be resolved again. This operation takes several minutes and may interrupt database connections. Therefore, you are advised to change a floating IP address during off-peak hours.
You can use an automatically-assigned IP address when creating a DB instance.
You can change the floating IP address of an existing DB instance.
An in-use IP address cannot be used as the new floating IP address of the DB instance.