This question has come up a couple of times on various forums on the Inter-webs so I figured I’d write up something a little more in-depth than I’d be able to do on a forum.
There’s no way to easily flip a standalone SQL Server instance to a clustered instance. However moving a standalone instance into a clustered instance isn’t that tough, but it will require some down time to the clients.
If you need to allow the clients to connect to the same name and IP address then this gets a little more complex, but is still easily do able. In this post I’m assuming that you don’t need to keep the name and IP address. I’ll post another post later on which covers that situation.
The easiest and least impacting way to handle this migration is to build the machine which currently isn’t doing anything as a single node SQL Cluster. This will let you build the machine in advance and let you handle the database migration from the standalone machine to the cluster. After you have build the cluster and brought it online, you can detach the database from the standalone machine and attach it to the clustered instance. Once you are sure that the clustered instance is working for you, you can uninstall the standalone instance. You can then add the machine to the cluster, then install the SQL Service on the second node of the cluster.