What are the risks of upgrading DB major version in replica and then promoting it as master?

Recently AWS decided to force update all MariaDB RDS under 10.3, and I am using AWS and my RDS are using MariaDB 10.1. We are trying to minimize the downtime and any risk of data out of sync.

Our plan is to create a replica and upgrade it from 10.1 to 10.5. After that we will let it to consume binlog from old non-upgraded RDS then promoting it as Master and point our website to the newly created and upgrade RDS.

Assume we will have continuous stream of data coming into the RDS, what are the risk of steps above? Is the new RDS able to get all binlog genereated during the upgrade time? We estimated there will be ~ 1.5 hours to finish upgrading from 10.1 to 10.5

Thank you for helping