RDS provides you with new versions of database software.
Generally, upgrade is not mandatory. The database of an RDS instance is upgraded only when you restart the RDS instance.
In rare cases such as critical bugs and security vulnerabilities, RDS enforces database upgrade during the maintenance period of the instance. Such mandatory upgrade only results in transient database disconnection without any obvious adverse impact on the application when the database connection pool is correctly configured.
You can change the maintenance time on the console or using API to prevent mandatory upgrade during traffic peak periods.