SANCopy is an EMC piece of software - and thus does not work with Nimble.
The easiest way to do what you're asking is by shutting an ESXi host down and modifying the BFS policy within Cisco UCSM to a new BFS volume on the Nimble array, and rebinding/rebooting the server once created. If done correctly it will take no longer than a few minutes to do so within UCSM. You can then roll this out one by one on the ESXi hosts.
You can also use tools from the likes of Datacore to move the data across should you wish - but BFS volumes are fairly transient and quite easy to recreate if needs be.
Here's an article which details the steps: How to move to a new array when using boot from SAN - TechRepublic
Copying a boot LUN across from one SAN vendor to another is almost certainly going to give headaches - and many vendors (like VMware) don't support the process of doing so.
You could use Auto-Deploy within vSphere 6 to automate the process which would be neat!
As mentioned above the best option is to recreate new BFS volumes on the Nimble array, and one-by-one change the boot policy within UCSM for each ESXi host. This will ensure now VM downtime although you will be powering off/on hosts by rotation.
If you follow the link I shared that will give you the steps.
You can go down the route of copying across the BFS data using something like Datacore but I can almost guarantee there will be problems in the process.
OR use VMware Auto Deploy.