We have 2 Hyperv Servers and a SAN. When they were configured, High Availability wasn't on the horizon. But it is now.
Can I convert the current servers in place to use Failover Clustering? Or Would I have to create a cluster with different hardware, migrate the current VMs over to them, and then bring in the older servers?
You can do it in place. You'll have to reboot one or more times, so there will be unavailability, but otherwise you can take a running Hyper-V server, add the Cluster Service, Create the AD Cluster and join the Servers without destroying the current configuration.
Side Note: VMs in the Cluster are not automatically highly available. You must explicitly "convert" them after the fact. This can be done in Failover Cluster Manager or SCVMM (if you have that).
I don't think you can do it in place but it's fairly straightforward. Once once you have the servers in a cluster simply move them in and make them highly available. Basically:
This MS article walks you through it: http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=12648