1. Does the LUN need to be resignatured for the new vCenter?
No, since will be the first time the new vCenter will see those LUNs.
2. What risks / caveats / gotchas are there to this scenario?
No many risks, cause even if you use HA on both vCenters, a new folder for each cluster will be created on datastore: With vSphere 5.0 and HA can I share datastores across clusters?
