Our SA2500 kicked the bucket out of warranty, and we quickly built up the virtual appliance to at least let 3 users in w/o a license. Is it possible to move a 50 user license from our now-deceased SA2500 to the virtual appliance? I know the virtual systems require a licensing server and can't have the licenses applied directly, just want to know if this is even possible.
No you cant move these licenses.
You can't use the licenses directly on anything but the platform they were purchased for.
But I would contact your Juniper partner reseller. There are generally programs you can use to get some credit for existing licenses when you buy a newer platform.