Although Cisco UCS has been around for a while now, it's still a new technology to many engineers and environments. I may be biased a bit here, but UCS does some unique and innovative things that other server vendors don't.
A common scenario I've seen lately is one in which a customer has a single Fabric Interconnect that is a few years old, and they want to improve the availability and performance of their UCS by replacing it with a pair of new Fabric Interconnects. (Let's call these FIs now; I'm tired of tapping out the full name.)
Of course, this is a well travelled path. The short answer is to do the following:
1) create a one-node cluster with your original FI.
2) add the first new FI to your cluster.
3) make the new FI the primary cluster node.
4) remove the original FI from the cluster.
5) finally, add the second new FI to your cluster.
Yes, that's the short answer. Long answer to be posted in the morning.