When you upgrade the flash using the expansion slot on the front, you do not need to copy anything as you are simply adding additional Compact Flash. Your original file system is still stored on the internal flash.

Now if you replace this internal flash (system flash) you need to TFTP your contents onto a TFTP server then, copy them back to the new flash.

http://www.cisco.com/en/US/docs/security/asa/hw/maintenance/guide/procs.html#wp1031632