What types of nics do you have? If you add a device serviced, say, by the em driver, the operating system might assign the new device as em0 and push your existing devices from em0 and em1 to em1 and em2. You'll probably need to look at the MAC addresses and use the CLI menu's (1) Assign Interfaces to straighten it out.
Before you start, take a backup in Diagnostics->Backup/Restore and worst case you should be able to pull the new card, restore from the backup, and be back where you were.