Hi,
Refer these documents that i think might help:-
http://docwiki.cisco.com/wiki/Migrating_from_Cisco_FWSM_to_Cisco_ASA_Software
http://www.cisco.com/c/en/us/td/docs/security/asa/migration/fwsm/fwsm2asasm.html
The Major difference would be with the Interface configuration from VLAN on FWSM to Physical interface on the ASA device.
Thanks and Regards,
Vibhor Amrodia