In order to upgrade to Multi-AZ, we will need to move your DB to a different cluster.
In order to achieve that, you should create a new multi-AZ subscription and then create a DB under this subscription. Once this part is done, there are two paths to go:
1. You can backup your DB and restore it to the new DB. This is the faster and recommended option.
2. We can live sync your DBs. This might take a few days to coordinate with our DevOps. In this case, the only downtime will be the time it will take you to change the endpoint in your application.